História do Microscópio
- Table of Contents:
Chapter 1: Early History of the Lens
Chapter 2: The First Compound Microscope
Chapter 3: Microscopes of the 17th Century
Chapter 4: Mechanical Improvements of the 18th Century
Chapter 5: The Achromatic Lens Dispute, .
Chapter 6: The Microscopes of the 19th Century
- The microscope has become one of the most recognizable symbols of science. This paper covers the early history of the microscope, starting with use of a simple lens in ancient times, to the first compound microscope circa 1590, up to the microscopes of the 19th century. Another “History of Microscopy” is maintained by Chris Jefferies in the U.K. which covers more recent (20th century) improvements. I encourage you to visit this page as a complement covering some later developments. This is not a complete history. A number of good books exist on the topic, and several are listed under the Scientific Books section of the “Scientific and Medical Antique Collecting System” which I also encourage you to visit. I great ly encourage anyone with corrections, disagreements, or suggestions to send me email (firstname.lastname@example.org). About the Images in this Document: The images used in this document are derived from many sources, such as the original historic documents, the National Library of Medicine’s History of Medicine WWW service, and other WWW services. A few are from sources as listed in the above mentioned book sections. While I am working to replace copyrighted pictures, I urge anyone who wishes to use these images to research the copyright status before using them in non-private work. Early History of the Lens: The first use of a lens is a bit of a mystery, but it’s now believed that use of lenses is more modern than previously thought. This chapter should give some insighs on how optical theory developed making microscopy possible. The Lanyard Lens: The above is the famous “Lanyard Lens” discovered at Nimrod by Lanyard, and datable to 721-705 BC. This was long thought to be the first example of a plano-convex lens. In the last century, however, studies of this object have shown that the curved surface is actually faceted, and not rounded, as one would have done to make a lens. Secondly, cloudy striae within the stone, which add an aesthetic quality, make it a poor magnifier. It is now generally agreed that this stone was actually just an ornament which fell away from it’s mounting. It’s now thought that lenses were not used nearly so long ago. Ancient Writings of Optics Several ancient manuscripts give us datable evidence of the state of refraction and optics throughout the ages. 2nd Century BC: Claudius Ptolemy:Described a stick appearing to bend in a pool of water, and accurately recorded the angles to within ½ degree for this relationship. He then very accurately calculated the refraction constant of water. 1st Century AD: Seneca: Described actual magnification by a globe of water. He wrote the following: “Letters, however small and indistinct, are seen enlarged and more clearly through a globe of glass filled with water.” 962-1038 AD: Arabian Scholar Alhazen: Wrote the first major optical work Opticae Thesaurus discussing not only optical principles, but described the anatomy of the eye, and how the lens of the eye focuses an image on the retina. 1267 AD Bacon Wrote definite descriptions of simple magnification in his Perspectiva of 1267: “Great things can be performed by refracted vision. If the letters of a book, or any minute object, be viewed through a lesser segment of a sphere of glass or crystal, whose plane is laid upon them, they will appear far better and larger. While the above exerpts give a glimpse at optical knowledge in the past, it’s important to realize that this knowledge was likely confined to the learned scientists and philosophers. It wasn’t until the invention of spectacles that optics came into common use. Early Evidence of Spectacle Use: It has been said that spectacles were in use in China well before they were known in the West. However, studies of early Chinese spectacles show that often the lenses were planar, without corrective abilities, and people may have actually used these colored glasses for cosmetic purposes rather than for astigmatism. There is one very early description of an isolated use of spectacles. Pliny the Elder wrote the following in 23-79 A.D.: “Emeralds are usually concave so that they may concentrate the visual rays. The Emperor Nero used to watch in an Emerald the gladatorial combats.” This quote appears to be the first description of using a monacle for correcting short-sighted vision. Strangely, even though this must have worked quite well, and many people must have read this passage, there is no other evidence of spectacle use for over twelve centuries. The Invention of Spectacles: The modern reinvention of spectacles occurred around 1280-1285 in Florence, Italy. While it’s uncertain who the inventor was, it is quite clear that spectacles quickly took hold into common use in that city, and use of them spread outward to the rest of the known world in just a few years. Considering the large percentage of people with visual problems, it’s not difficult to understand why there was such enthusiasm. Strangely, it’s not certain who in Florence made the first spectacles. Some give credit to a nobleman named Amati who died in 1317. It has been said that he made the invention, but told only a few of his closest friends. The First Compound Microscope: The story of the first “compound” (more than 1 lens) microscope is an interesting one. Much is unknown, yet many things are known. With the lenses of spectacles widespread, and their obvious magnification properties, it was only a matter of time before someone put two together to make the first compound microscope. Indeed, this was probably already happening with telescopes just before this as Dutch Spectacle makers were experimenting with multiple lenses. Since a microscope could be made by just reversing a telescope, this may be where the idea originated. There is a terrific amount of mis-information about who invented the microscope. Fairly respectable references have said Gallileo invented it shortly after inventing the telescope. This is not so, as Gallileo didn’t purchase his first telescope until around 1607. Many people think that Leeuwenhoek invented the microscope. This is also very untrue, as while his microscopes were very simple and crude, he started making them long after very elaborate models were available and many important discoveries had been made by them. Zacharias Jansen Credit for the first microscope is usually given to Zacharias Jansen, pictured above, in Middleburg, Holland, around the year 1595. Since Zacharias was very young at that time, it’s possible that his father Hans made the first one, but young Zach took over the production. Details about these first Jansen microscopes are not clear, but there is some evidence which allows us to make some good guesses about them. The below early microscope found in Middleburg, Holland, corresponds to our expectations of the Jansen microscopes. Early Microscope Attributed to Jansen Unfortunately, no early Jansen microscope has survived to the present. The above instrument surfaced in Middleburg, Holland, in the 17th century, and was reported to be an early Jansen made microscope. That microscope now resides in a Middleburg museum. Most historians have come to doubt the origin of the above microscope, but because it corresponds well with the known description of the Royal Jansen microscopes, it is at least very similar to the Jansen microscopes. The Royal Jansen Microscopes Luckily, there was one true Jansen microscope which survived long enough to be studied. As was customary at the time, the Jansens made several versions of their new invention to give to royalty. We know that they sent one of their microscopes to Prince Maurice of Orange, and one to Archduke Albert of Austria. While neither of these instruments survived to modern times, the later of them was preserved until the early 1600’s, when a Dutch diplomat and childhood friend of Zacharias Jansen named Cornelius Drebbel, examined it and recorded his observations. We would expect a royal presentation microscope to be more ornate and well-made than usual, yet it should retain the same mechanical configuration. Drebbel described the royal instrument as being composed of 3 sliding tubes, measuring 18 inches long when fully extended, and two inches in diameter. It was very ornate, with 3 brass dolphins at the end, forming the feet of a tripod. Besides the ornate tripod, this description is quite similar to the microscope found in Holland. Optics of the Jansen-style Microscope: The above diagram shows the optics of the Jansen-style microscopes. Note that it contains a 2 lenses, and diaphragms between the tubes to cut down on glare from the crude lenses. The microscope at the Middleburg museum was said to have a magnification of 3X when fully closed, and 9X when fully extended. Probably the first major improvement in microscope optics was the introduction of a 3-lens system. This happened early, possibly by Robert Hooke (described later), by using a 2-lens “Huygens” eye-piece which was in common use on telescopes. The Rapid Spread of Knowledge: After the Jansen invention, word traveled rapidly throughout the known world. Within just a few years, there were many microscope makers throughout europe, and learned men such as Galileo were using them. Microscopes of the 17th Century: The seventeenth century was a period of great interest in microscope, as well as some of the earliest discoveries. The word “Microscope” was first coined by members of the first “Academia dei Lincei” a scientific society wich included Galieleo. But the microscope wasn’t just a scientific tool. Throughout this century and Victorian times, microscopes were owned by the upper-class as recreational toys. Three Lens System As mentioned before, the first technical advancement of the Microscope after Jansen was revision of it’s optics from the 2-lens system to the 3-lens system, early in the century. In the above diagram, B is the eye-lens, D is the Field lens, and F is the objective lens. The eye views at point A. It has been said that Robert Hooke was the first to use this, by using the 2-lens “Huygens” eyepiece which was standard for the telescope. This allows for better conservation of light from the object, while avoiding a very large eye-lens from which the eye must be held an uncomfortable distance away. This three-lens system remains the standard configuration of microscopes today, except that that each lens may be made out of a combination of close lenses. Why the Utility of Microscopes was Questionable: In the early 17th century, a few papers were published on microscopic findings, but the first two important papers weren’t until 1660 and 1665, when Marcello Malpighi proved William Harvey’s blood circulation theories, and Robert Hooke’s wrote his “Micrographia.” The significance of these works may be difficult to understand today. It wasn’t clear back then that the microscope would ever be useful to make scientific discoveries. Some thought that although it was interesting to look at the tiny legs on a flea or parts of other insects, you couldn’t really make any new discoveries with such a device. It wasn’t appreciated that by looking at things up close, you would be able to see anything really different than at a distance. This thought can be understood by considering examination of “milk” under a microscope: nothing really new becomes visible—there just isn’t any structure which becomes apparent at the magnification involved. These early thinkers had no idea that a very important structure lies in all living tissues that was within the reach of light microscopes: The Cells. Marcello Malpighi, circa 1660 Marcello Malpighi was one of the first great microscopists, and even today is considered the father of embryology and early histology. His first discovery with the microscope was of monumental importance in animal physiology. Just 21 years earlier, William Harvey stunned the academic and medical environment in the publication of his “De motu cordis & sanguinis in animalibus” in which he presented experimental and logical proof that the long-held theories of Galen were wrong, and that blood actually travels in a circular motion from the heart, around the body, then back to the heart. [Since the time of Galen, it was thought that the blood is produced from the intestines, travels to the liver, then the heart, and then is distributed to the body by both veins and arteries, where it is consumed.] In this time such a radical idea should have been met with banishment, yet Harvey’s important position as a top physician to royalty, as well as his meticulous theoretical and logical proof, made the world stop to consider it. But, as convincing as his argument and evidence was, his theory required some form of connection between the arteries and the veins, which nobody could see. In 1660, 3 years after Harvey’s death, Marcello Malpighi used a microscope to see the capillaries, the microscopically thin blood vessels which formed the needed connection between the arteries and veins. For the next 200 years, “Fish-Plates” (brass curved plates with which one would tie down a small fish) became standard equipment for microscopes. This allowed viewing of corpustels (red blood cells) traveling through the transparent tail capillaries. This interest illustrates the importance of the capillary discovery. 17th Cent. Italian Models as Malpighi Likely Used: The above two microscopes are of the type made in Italy, around the time Malpighi did his work. These are small hand-held items, maybe 5-8 inches tall - quite a contrast to the 2-foot tall instruments being made in England around the same period. The left-most item was made by Giuseppe Campani (1635-1715) and the right-most item was made by Eustachio Divini (1610-85.) A great deal has been written on Malpighi’s life, and it is not completely certain which microscopes he used, but his personal papers do mention purchase of a Divini microscope, and the item on the right is often considered to be the type used by him. Italian Microscope as Galileo May have used. The above microscope is another simple Italian style from the early 1600’s, which is said to be the type Galileo used. It is interesting to note that Italian microscopes remained of this smaller, simpler type for some time, while the English microscope makers were rapidly making two foot tall monsters with many mechanical innovations. Robert Hooke’s Micrographia, 1665 Robert Hooke was a mechanical genius who graduated from Oxford, and worked with Robert Boyle in his famous gas experiments. Robert Hooke’s Micrographia was an important milestone in proving the importance of microscopy. The following image of cork is probably the classic example of Hooke’s accomplishments. Hooke’s Drawing of Cork. Before Hooke’s time, it wasn’t known why cork had the unusual properties that it had: it was very light, and could float well on water, and it was firm, yet could compress under force. When Hooke looked at a thin slice of cork under the microscope, it became clear why cork had these properties. He could see that the substance was mostly air, with pieces of material making up a mesh-work of supporting structure around the tiny air pockets. Hooke named these pockets of air “cells” after the small monastery rooms they reminded him off. While he didn’t at the time understand that these air “cells” were the shadow remains of what is now considered a cell, the name remains. Hooke’s Microscope by Cock, 1665 The microscope used by Hooke was illustrated in his paper. Hooke did not make his own microscopes; they were made by London instrument maker Christopher Cock, whom Hooke gave much advice on design. In return, the success of Hooke’s book made Cock a very famous microscope maker, and popularized the side-pillar design. These were large instruments, nearly 2 feet tall. The very large body tube was attached to the stand by a screw, so by rotation, an object could be brought into focus. The object was placed on a pin on the lower stage, and light illuminated the object from above. The shown illumination apparatus, an oil flame and a globe and lens to focus the light, has been said to be made by Hooke himself. Obviously with the weight of a 2 foot tall body tube, and the crude screw threads that were possible at the time, there was a great deal of wobble associated with screwing the focus up and down. Later models made by Christopher Cock had a much wider nose to increase stability. Below is slightly later version, again made by Cock, which has a wider nose screw. Another Microscope by Cock, after 1665. Several other examples exist in such collections as “The Billings Microscope Collection” and the “Royal Microscope Society Collection.” These are usually very beautiful instruments, using bright and ornate tooling along the leather tube covers, brass uprights, and Lignum Vitae, a very hard black wood. John Yarwell and the English Tripods Later in the 17th century, microscopes made a morphological change. Probably due to the instability of the thin side-pillar and small screw-nose holding the huge tube on the Cock/Hooke style of microscope, the English Tripod became popular. Tripod Sketched an 1631 Journal The earliest evidence of an english tripod style microscope was from the Journal of a Dutch schoolmaster, Isaac Beeckman, who draw the above in his diary in 1631. Tripod in Yarwell’s 1683 Trade Card. John Yarwell, who was probably the next great microscope maker, showed the above microscope in his trade card of 1683. It is similar in size, body tube, and focus to the side-pillar model of Cock, and the object lies flat on a small platform. The improvement was the more sturdy tripod holding the large tube straight up and down. Another 17th Century Tripod, 1680’s Many English-Tripod-Style microscopes were made in the 1680’s, but very few exist today, confined mostly to museums. The above is a fine example. Leeuwenhoek and the Use of Simple Microscopes: Antoni van Leeuwenhoek (circa 1670) In the later part of the 17th century, something quite unexpected happened in microscopy. Using tiny single-lens microscopes of his own design, this Dutch Draper/amateur scientist, started making incredible discoveries. He began writing letters to the Royal Society in London in 1673, which were published in Philosophical Transactions. He described experiments which could be performed with simple microscopes, and made exciting drawings of his microscopic world. Leeuwenhoek make the first descriptions of protozoa, bacteria, and spermatozoa which he called “animalicules” and made the first detailed descriptions of the red blood cell. It would seem incredible that an amateur could out-do the scientific community using simple instruments of his own making, but the reasons Leeuwenhoek was successful are simple. The lenses in common use in the 17th century were quite crude, often made by smashing molten glass between pieces of wood. Because of optical problems such as chromatic and spherical aberration (described later) the images at modest magnifications (over 40 or 50X) were blurry, with colorful halos. When multiple lenses are used, this error is synergistically increased. By using only a single, high-power, quality lens, Leeuwenhoek found he could get much clearer images than with compound microscopes. A recent study of the remaining Leeuwenhoek microscopes shows their magnifications to be from 50X over 200X, with resolutions as good as 2 microns. Until around 1800, the compound microscopes could only resolve as well as around 5 microns. A Leeuwenhoek Microscope This diagram shows a microscope by Leeuwenhoek. It is about 3-4 inches high, and made of two brass plates riveted together. A small hole is made in the plates, between which the very tiny but high-power lens is placed. On the back side, a screw with a needle provides a place to place the object, and allows crude focusing. How a Leeuwenhoek Microscope is Used: This photo shows how a Leeuwenhoek microscope is used. After attachment of the object, the entire microscope must be held very close to the eye. This is not easy to do, as the faint image is hard on the eyes, and focus of the crude device requires great patience. But for the price of convenience, the images obtainable were greatly superior to the compound microscopes. Robert Hooke, author of the early Micrographia, said in his Microscopium lecture of 1679: I have found the use of them [simple microscopes] offensive to my eye, and to have much strained and weakened the sight, which was the reason why I omitted to make use of them though in truth they do make the object more clear and distinct, and magnify as much as the double [compound] microscopes; nay, to those whose eyes can well endure it, it is possible with the single microscope to make discoveries much better than with a double one, because the colours which do much disturb the clear vision in double microscopes is clearly avoided and prevented in the single. Leeuwenhoek’s Secret Lenses: Leeuwenhoek’s method of making the tiny, high-quality and high power lenses was kept secret. A study has recently been done on the few remaining copies of Leeuwenhoek’s microscopes, and it appears that some of the lenses may have been made by grinding, while the best ones were blown. Leeuwenhoek learned that when a glass bulb is blown, a small drop of thickened glass forms at the bottom of the bulb (much like a drop sits in the bottom of a blown soap bubble.) By carefully breaking away the excess glass, this tiny drop can be used as a lens. The Mystery of the Leeuwenhoek Microscopes: In 1747, two years after Leeuwenhoek’s death, over 500 of these small microscopes were auctioned off, and 26 special silver microscopes were given to the Royal Society of London. Of this very large number, only nine microscopes are known to exist today. Even the 26 silver instruments which became part of a large and famous collection somehow disappeared. The nine Leeuwenhoek Microscopes are priceless—often they are kept in a museum safe and only a replica is on display for the public. As the years go on, it becomes less likely that another specimen will turn up, but forgeries have turned up and will likely continue to surface. The Simple Microscope Remains Important: After Leeuwenhoek’s incredible successes, simple microscopes regained their place as complements to compound microscopes. In fact, up until the early 19th century, some of the best microscopes could be used both as simple or compound microscopes. When George Adams made a presentation microscope for King George III at the end of the 18th century, it was actually two complete microscopes attached back to back: a simple microscope, and a compound microscope. Mechanical Improvements of the 18th Century: The 18th century was a time of several mechanical improvements to the microscope, increasing stability, facilitating smooth focus, and other ease of use issues. This was also the century that the telescope, the microscope’s slightly older brother, improved optically with the discovery of achromatic lenses. The microscope, unfortunately, did not benefit significantly from that discovery during this century. John Marshall’s Microscope of 1704 By 1690, the two leading microscope makers were John Yarwell (discussed in chapter 3) and John Marshall. Both makers were trying to capitalize on Malpighi’s discovery of capillaries in 1660, by making new models and advertising them for observing the circulation of blood through the capillaries in fish tails. Marshall scored a great advantage by having his microscope advertised in the very first technical dictionary, Harris’s Lexicon Technicum which was published in 1704. The advertisement is pictured above. The book became an incredible success. This is another example of successful book popularizing a microscope style (just as Hooke’s Micrographia advanced the Cock style.) The above is the actual advertisement in the Lexicon Technicum. Note that the body tube can be rotated upon the pillar so that the base is at the side. Then a small glass “stage” can hold a fish, and with a light below the stage, the blood flowing through the capillaries can be viewed. The later invention of a sub-stage mirror would have eased this. Another Marshall Style Scope Here is another Marshall model. Note the focus and positioning options: the body tube can be slid up and down the pillar by loosening a screw, and the stage can be raised and lowered. Fine focusing is by a screw to the side of the pillar which will slowly raise and lower the tube. There were 3 stage options: a stage for opaque objects, which had a bulls-eye lens to focus light from above, a glass stage for transparent objects (like the fish) and a bonanni spring stage as shown, which allowed sliders, of ivory or wood. These models were very popular, and may occasionally be found in the large auction houses, one selling recently for around $30,000 U.S. The Culpeper Tripod, circa 1725 The next widely popular microscope style was the Culpeper Tripod microscope, invented by Edmund Culpeper around 1725. This is a direct modification of the English Tripod popularized in the 1690’s, but several changes made it much easier to use. The most important change was that the opaque object stage of the Tripod was raised up, then a hole was put in it, and a mirror on a gimble was placed below the hole. This allowed ease of illumination using available light, when examining transparent objects. Later (Mid-19th Century) Culpeper Design. The Culpeper style microscope was a popular style for over a century. During this time it’s shape evolved somewhat. First, the straight brass tripod legs were replaced by S-shaped legs (this modification attributed to John Adams Snr., a maker described later.) Next, the composite body tube was replaced by brass body tube on a wooden base, then a rack and pinion focus mechanism was added in the early 19th century, and then wood base was replaced with a brass plate in the later versions. The above example is a probably circa 1830-50, in the standard pyramid case. Benjamin Martin’s “Drum” Style, 1738 Another new design by Benjamin Martin of London was introduced in 1738. This was a small simple microscope where the microscope body tube is enclosed by an outer tube. There were cut-outs in this outer tube where the object could be placed on the flat stage under the objective. The inner body tube could then be slid up or down in the outer tube for focus. Martin never used the term “drum” in describing this microscope. He called it a “Pocket Reflecting Microscope” (reflecting referred to the light reflecting up from opaque objects.) He made the scopes out of brass, lignum vitae (a hard black wood) and rayskin. Later versions around 1760’s had small angled mirrors below the stage to allow viewing of transparent objects as well as opaque. Later, Mid-19th century Drum Style. Strangely enough, the peak period of popularity for drum microscopes was in England from 1820 to 1860. This became a very common, inexpensive style, selling in great numbers. These newer microscopes were larger, made entirely of brass, had either rack and pinion or slide focus, and had an adjustable mirror below the stage. Some of the high-quality microscope makers in Paris, such as Oberhauser, adopted the drum style for some of their highest quality models in the 1840’s and 1850’s. Late in the 19th century, and well into the 20th century, many french makers were producing small and cheap drum-style microscopes as toys and for amateurs. These were available in catalogues such as “Sears Roebuck and Co.” as late as the 1920’s, and are often seen at flea markets and antique shops for $50-$100 U.S. John Cuff’s Microscope, Circa 1742: The next style to become popular was the Cuff style. Some have called this the first microscope offering some ease in use. This microscope had a much smaller body tube than earlier styles, which allowed more stability. A strong side-pillar holds the tube and stage firm, with easy access to the stage from all sides (unlike the Culpeper.) Focus is by a fine screw which slides the body tube up and down smoothly. The stage is cut away in shape allowing more flexibility of accessories and view of awkward objects. The real reason this model stood out from the competition of the time was publicity from a popular book - just like Micrographia did for Cooke’s scope, and Lexicon Technicum did for Marshall’s scope. In this case, Henry Baker published a book entitled The Microscope Made Easy in 1742. The book devoted an entire chapter to John Cuff’s inventory of available microscopes. The book was a great success, quickly selling out its first edition, and going through several printings. The English instrument trade quickly began to copy Cuff’s style, as demand for it was high. George Adams Snr., and His Microscopes, (circa 1746) George Adams became the greatest microscope maker in the second half of the 19th century, largely by plagiarizing his competition’s book. In 1746, just 4 years after Henry Baker published The Microscope Made Easy George Adams wrote a competing book Micrographia Illustrata. Baker was not happy about this, because George Adams basically copied most of his work, redrawing some of the plates, and adding very little. In this heavily plagiarized book, Adams introduced several of his new models, such as the above “New Universal Double Microscope.” This was one of the first examples of an objective changer. This microscope could be used as both a simple microscope by removal of the body tube, and a compound microscope (the reason for “universal” in its name.) The focus screw is in the base, and runs up the pillar to the movable stage. Adams also introduced another objective change method, where instead of a rotary wheel, a long brass slider with several objective lenses in it could be slid into the nose of the microscope for different magnifications. George Adams Snr became a premiere microscope maker, and he became the instrument maker to King George III, making a huge, ornate, silver microscope having a simple microscope on one side, and a compound microscope on the other. Solar and Projection Microscopes, circa 1740. Chapter 5. The Achromatic Lens Dispute, and other Optic Improvements: Before continuing the story of the morphological evolution of the microscope, it’s important to pause and discuss early optical limitations and how they were overcome. While the 18th century produced some great mechanical improvements for the microscope, making it much more sturdy and easy to use, the images obtainable remained rather blurry with colorful halos around objects. This was largely due to the problems of “Chromatic and Aspheric Aberration.” The reason the single lens “simple” microscopes remained important throughout the century was that a single lens system has much less aberration because the distortion becomes synergistic with multiple lenses. This allowed simple microscopes to attain around 2 micron resolution, while the best compound microscopes were limited to around 5 microns. The Problem of Chromatic Aberration The problem of chromatic aberration is shown above. Any substance which can bend light (such as the glass in a lens) will bend light of different wavelengths (colors) slightly different amounts. This leads to the fact that any simple lens will have slightly different focal distances for each color. If an object is white (composed of all colors) as shown above, the red component will come into focus in a different place from the blue component. As a result, when we focus on a object, it will have a blurry red or blue halo around it. The Secret Invention of the Achromatic Lens The solution to this problem came in the 1730’s, when a barrister named Chester More Hall noticed that a newly created glass, “Flint Glass”, seemed to disperse the colors more than the older “Crown Glass” did at the same magnification. (the achromatic lens) He hypothesized that if he used a concave lens of this new glass right after the old crown glass, he could pull the different colors back into alignment, without losing all of the magnification of the first lens. Thus, the achromatic lens was born. Chester More Hall realized the importance of this discovery, as telescopes were big business in England at this time, so he knew he had to keep his idea secret until he tried it out. Therefore, he contracted with two different optical shops to make the crown glass lens and the flint glass lens for him, so they wouldn’t suspect what he was up to. Unfortunately for him, both lens makers subcontracted the job out to the same lens maker, George Bass, who put 1 and 1 together, and realized what Hall was trying to do. While Hall undoubtedly produced the first achromatic telescope in the 1730’s, he never publicized his invention, nor took out a patent. John Dolland Learns the Secret, and Patents it. The lens maker, George Bass, kept word of the two lenses secret for 20 years, until in the 1750’s he met John Dolland, another telescope maker who was experimenting on reducing chromatic aberration. [John Dolland was the father of Peter Dolland, who became an important microscope maker later in the century.] After hearing from Bass about the two lenses, Dolland understood that this was the answer, and by 1759, Dolland succeeded in making an achromatic lens. What’s more, he took out a patent over the invention, which made him a terribly wealthy man. The English telescope makers were not happy about having to pay royalties to John Dolland for every new achromatic telescope they produced. When word got out that Dolland’s invention was actually invented 20 years earlier, they disputed the patent rights. The courts decided that even though Hall may have made the discovery, a secret discovery would do the world no good. Dolland was the man who brought the discovery to benefit the world, so the patent was upheld. Benjamin Martin may have made the same discovery. There is good evidence that Benjamin Martin (of drum-microscope fame, and a well known microscope maker) made the same discovery of the achromatic lens independently and before Dolland. Unfortunately, Martin never got a satisfactory lens made, and gave up on the idea. Benefit for the Microscope Comes Slow. While the achromatic lens was an immediate success for the telescope, the tiny objective lenses of the microscope were much harder to make in an achromatic style. Because of these difficulties, the first practical achromatic microscope objectives weren’t available until around 1800, and it was even later before they were commonly available. Spherical Aberration After the chromatic aberration problem was solved, there was still the problem of spherical aberration. This is a distortion caused because light from the object which hits the edge of the lens does not have precisely the same focal distance as light which comes through the center of the lens. The problem can be made better by using very small apertures or diaphragms to limit light angles (which we learn later from Ernst Abbe reduces resolution) or by making the lenses with less curvature (which limits our ability to make higher power lenses.) If they used multiple lenses to increase magnification power, the error in the different lenses becomes synergistic, and the image gets worse. Joseph Jackson Lister Solves the Problem in 1830 The problem was finally solved in 1830, by Joseph Jackson Lister (father of Lord Joseph Lister, the surgeon who discovered antiseptic technique.) In a paper published in the Philosophical Transactions in 1830, he showed mathematically how if multiple low-power lenses are placed a certain precise distance away from each other, you will still get the spheric aberration of the first lens, but you will not get any added component from the following lenses. Since the amount of aberration in the first lower-power lens could be made minimal, the aberration of the whole series would be very small, even though magnification power was high. The Enthusiasm from Microscope Makers is Subdued: Lister was a microscopist, not a microscope maker. After Lister published his paper, he sat back and waited for the London makers to produce the new objectives. After several years without anyone making his improved objective style, he decided he was going to have to build a scope himself. A Tulley/Lister Corrected Lens Microscope, 1830’s Lister worked with Tulley and Sons to produce this microscope in the mid 1830’s. The additional supports in the back were necessary for stability. Andrew Ross’s Lister-Limb, 1837. A few years later the first major manufacturer produced a microscope according to Lister’s improved specifications. For stability, Ross produced a very thick brass support frame of triangular cross-section, and all optical parts (tube, stage, condenser, mirror) slid along this line. This strong curved frame became known as “The Lister Limb” as it was made specifically for the new Lister scopes. Adjustable Objective by Ross, circa 1840 It’s interesting to note that the Lister-corrected objectives needed extremely tight tolerance of lens position for proper image correction. In fact, the presence or lack of a coverslip, or even different thicknesses of coverslips, would throw off the precise optical correction, and degrade the image. Because of this, early Lister-style objectives usually had a slide or rotating adjustment on them which would need to be placed in one position if no coverslip was used, and the other position if a coverslip was used. An example of these objectives is above, where rotation of the objective would slide the optics to different alignments. In the mid 19th century it wasn’t even possible to manufacture coverslips to a standard thickness, so it became necessary to measure your coverslip with a micrometer, and then set the adjustment on your objective to match it. In modern times the thickness of the coverslip has the same impact, but today, instead of having an adjustment on the objective, is that coverslip thicknesses have been standardized, and today’s objectives assume a that a coverslip will always be used. Other Attempts to Avoid Aberrant Lenses Besides using simple (one-lens) microscopes, there was another way to eliminate the achromatic problem in lenses. As reflecting telescope makers had learned, the optical focus could be performed with curved mirrors instead of curved glass. Mirrors have the advantage that reflection of light off of their surface is more straight forward: the angle of the “incident” light hitting the surface will be exactly equal to the angle of the “reflected” light from it. It doesn’t matter if you’re using red or blue light - their reflected angle will be exactly the same. A Reflecting Microscope, 1820-1840’s. Above is an example of a reflecting microscope. Italian professor Giovan Battista Amici first produced this type in 1813, just after making the reflecting telescope. He was soon joined by Chevalier of Paris, John Cuthbert of London, and many others. In principle, reflecting microscopes should be free of chromatic aberrations, but they are very difficult to fabricate and use. Since this was a period of rapid improvement of lenses, the style didn’t compete very well with lenses past around 1840. Rapid Empiric Progress 1820-1850 With the Achromatic and Lister-corrected objective invented, progress in resolution of microscopes finally became a concern. Early in this century the microscope makers developed standardized ways to compare resolution of objectives every time they made a new style. They found a number of objects in nature, such as diatoms, which have repeating structures near the resolution of their objectives, and so by looking at these objects with different objectives, it became clear when one had slightly better resolution. Abbe’s Formula, Not Known Until 1877. The problem that these makers didn’t know, was that even with chromatic and spherical aberration solved, there is one more factor involved with making a microscope as good as physically possible: angular aperture. While it wouldn’t become proven and well understood until Ernst Abbe publishes a paper in 1877, physical laws dictate that the minimum resolving distance (d) is related to the wavelength of light (lambda) divided by a number known as the Numeric Aperture, which is proportional to the angle of the light cone (theta) formed by a point on the object, to the objective. Stated in simpler terms: in order to get the maximum amount of resolution from a microscope, the objective must collect as as large of a cone of light as possible from the object. Empiric Increase in Resolution Even though the above formula would not be known for many decades, just by randomly making various objective designs, then keeping the designs which seemed to have better resolution, the resolution of microscopes improved greatly. By the 1850’s, the major makers had reached the theoretical limit of resolution without an immersion (oil) objective. Hence, by this time, microscopes could distinguish two points as different points if they were at least 0.28 microns away from each other. In effect, what they were doing was maximizing the angle of the light from the object to the objective, but they didn’t necessarily realize why this was helping. It was in the 1870’s, when experimenting with water immersion objectives (which increase the n in the above equation), Ernst Abbe, working for the German maker Carl Zeiss, elucidated the formula for which he is famous (and brought Zeiss to the forefront in microscope technology.) The Theoretical Maximum is Finally Reached By the 1880’s, using oil immersion objectives, a Numeric Aperture (N.A.) of 1.4 had finally been reached, allowing light microscopes to resolve two points distanced only 0.2 microns apart. With the exception of some very unusual immersion fluids or ultraviolet light, this remains the limit today. Chapter 6. The Microscopes of the 19th Century The nineteenth century was a great time for the microscope. Microscope makers were finally working on the quality of the optical image, instead of just striving to make mechanical improvements as they had throughout the 18th century. The optical problems of the past were corrected through the work of Dolland and Lister by the 1830’s, so that finally large clear images were being produced, instead of large blurry images. By the 1850’s, the maximum theoretical resolution with an air objective (0.28 microns, from an N.A. of 1.0) had been reached. The oil immersion objectives pushed this limit (to 0.20 microns) soon afterward. A wide selection of microscopes were being made by the 1850’s. The leaders in quality were the three English firms of Andrew Ross, Powell & Lealand, and R. & J. Beck. Other English firms such as Swift & Son and Watson & Sons became very important in the later century. Cary was making inexpensive, and portable naturalist models. The Drum-style was reborn. The French firms Chevalier, Oberhauser, and Nachet became well known. Later in the century, the German firms of Carl Zeiss and Leitz became an important supplier. This was also the century that America began to make microscopes. This started with Charles Spencer in 1840, who was known to make objectives at least as good as the English firms, at a more reasonable price. The English Firms Andrew Ross’s Lister-Limb, 1837. Andrew Ross made history by being the first major maker to produce a microscope according to Lister’s specifications which reduce spherical aberration. He was making other microscopes before that, but it was his association with Lister from 1837-1841 which made him a leader. Smith (below) and Tulley (last chapter) also made early microscopes in association with Lister, but neither of these companies made any significant quantity. Ross Style Introduced 1843 This is Ross’s most famous stand, introduced in 1843 which has a triangular bar-shaped limb, and a Y-shaped foot. This style was copied extensively by most of the microscope makers for the rest of the century. Smith & Beck’s Style early 1840’s James Smith produced an early instrument for J.J. Lister in 1826, and he went into partnership with Lister’s nephew Richard Beck, in 1847. In 1851 Joseph Beck joined his brother’s firm, and then Smith retired in 1865. This company (named in succession “James Smith”, “Smith & Beck”, “Smith, Beck & Beck”, then “R&J Beck”) produced some very nice microscopes, many of which were in the style shown above. Powell & Lealand’s stand introduced 1843 Hugh Powell made microscopes under his own name in 1840, then went into business with Peter Lealand in 1842. Their most famous stand which formed the backbone of their business for over 50 years is shown above. This microscope came in several different sizes, “No. 1” was the largest, and “No. 4” was a small portable. The triangular bar-limb was copied from Ross, but the tripod-legs were completely new. Many people considered the Powell and Lealand instruments to be the best in the world in the middle of the century. Spencer’s large “Trunnion” (circa 1855) In the United States, Charles Spencer began making microscopes around 1840. He became quite good at it, making achromatic objectives which were said to be at least as good as the English makers. Above is one of his large Trunnion microscopes from around 1855. This had a number of mechanical improvements. The company was slowly joined by many other makers in the U.S. which are outlined quite well in Donald Padgitt’s A Short History of the Early English Microscopes. In fact, there were 20 makers in the U.S.A. by 1880, but this fell to two by 1902 (Spencer and Bausch & Lomb). Spencer Lens Company stayed in business for over 100 years, until it was bought out by American Optical Company in the 1940’s. Zeiss’s Stand VI, circa 1880. Carl Zeiss of Germany began domination of the microscope market late in the century because of important improvements. With Ernst Abbe (discussed in the previous chapter) they pioneered the mathematical relationship between resolution and aperture, exploited the use of immersion lenses, and developed “apochromatic” lenses which reduced chromatic aberration even more. This was the era of the “Continental Style” frame which became the dominant style at the later part of the century. This style has the horse-shoe shaped base, and a fine-focus control on the back of the arm. Most makers were using this style well into the 20th century. Another German maker, “Leitz” began delivering quality microscopes around the same time as Zeiss, and survived the hard times that put the majority of makers out of business at the turn of the century. In the early 20th century, Leitz was selling a great number of microcopes annually, and was probably the major source of microscopes during this time. Movement from Brass to Iron The above photo also shows the movement away from all-brass instruments. Brass is an expensive metal, and it is very time consuming to polish it to a high-shine and then lacquer it. Later in this century, as competition made cost a more important concern, microscope makers started to paint much of the brass paint black rather than polish and lacquer it. Later, they began making the black areas out of iron. By the 20th century, it became common for most of the microscope to be made of black iron frames, with only the slides, tubes, and mechanical parts made of brass. Even later, it became common to nickel plate the remaining brass pieces, which protected them against corrosion better than lacquer, and was less expensive than the complex lacquering process. Other Styles Common in the 19th Century Gould or Cary style (1820-1850) The Gould-style microscope (also known as the Cary-style microscope) was a small, portable microscope in wooden case, which assembles on top of the case. These were popular naturalist microscopes, available from the 1820’s to around 1850. Gould worked for William Cary, one of the best known scientific instrument and globe makers. Some versions were very tiny (as this one) which would fit easily in a pocket. Others were much larger, having several accessories in the case, and with a round bracket on top of the case where the instrument assembles. Many different makers copied this design, but the ones signed “Cary” are common. 19th Century Drum (1820’s-60’s) The Drum style microscope, invented by Benjamin Martin in the 1730’s, and discussed in chapter 4, enjoyed a resurgence in England, peaking in the 1820’s to 1860’s. These were usually unsigned, being made by many makers for the trade. They were usually inexpensive, lower quality models for students and hobbiests. In France, some of the high-quality instrument makers in the 1840’s and 1850’s, such as Oberhauser, adopted the drum-style and outfitted it with top quality optics and accessories. By the late 19th century, many French makers were making small unsigned models or low quality as toys and for hobbiests. These continued to be sold, in two basic styles (single pillar from stage to body tube or dual struts) well into the 1920’s, and turn up often at antique shops. Summary: By the end of the 19th century, the beauty and craftsmanship associated with the microscope was being replaced by high-volume, low-cost, mass production. There remains interest in collecting the later microscopes, but their black and chrome features became more conformist than innovative. This history doesn’t pretend to be complete. It could never replace the descriptions, explanations, and pictures of a book such as Gerard L.E’ Turner’s Collecting Microscopes but it hopes to expose the reader to a few of the more common styles,
- makers, and obstacles which made the microscope the important machine it is today.
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