Table of Contents: Introduction 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 (email@example.com).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 longthought to be the first example of a plano-convex lens. In the last century, however, studies of this object have shown that thecurved surface is actually faceted, and not rounded, as one would have done to make a lens. Secondly, cloudy striae within thestone, which add an aesthetic quality, make it a poor magnifier. It is now generally agreed that this stone was actually just anornament which fell away from it’s mounting. It’s now thought that lenses were not used nearly so long ago.Ancient Writings of OpticsSeveral 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 recordedthe 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 onlyoptical 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 beperformed by refracted vision. If the letters of a book, or any minute object, be viewed through a lesser segmentof 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 likelyconfined 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 earlyChinese spectacles show that often the lenses were planar, without corrective abilities, and people may have actually used thesecolored 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 thismust have worked quite well, and many people must have read this passage, there is no other evidence of spectacle use forover 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 theknown world in just a few years. Considering the large percentage of people with visual problems, it’s not difficult to understandwhy 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 in1317. 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 areknown.With the lenses of spectacles widespread, and their obvious magnification properties, it was only a matter of time beforesomeone put two together to make the first compound microscope. Indeed, this was probably already happening withtelescopes just before this as Dutch Spectacle makers were experimenting with multiple lenses. Since a microscope could bemade 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 saidGallileo invented it shortly after inventing the telescope. This is not so, as Gallileo didn’t purchase his first telescope until around1607. Many people think that Leeuwenhoek invented the microscope. This is also very untrue, as while his microscopes werevery simple and crude, he started making them long after very elaborate models were available and many important discoverieshad been made by them.Zacharias JansenCredit for the first microscope is usually given to Zacharias Jansen, pictured above, in Middleburg, Holland, around the year1595. Since Zacharias was very young at that time, it’s possible that his father Hans made the first one, but young Zach tookover the production.Details about these first Jansen microscopes are not clear, but there is some evidence which allows us to make some goodguesses about them. The below early microscope found in Middleburg, Holland, corresponds to our expectations of the Jansenmicroscopes.Early Microscope Attributed to JansenUnfortunately, 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 Middleburgmuseum. Most historians have come to doubt the origin of the above microscope, but because it corresponds well with theknown description of the Royal Jansen microscopes, it is at least very similar to the Jansen microscopes.The Royal Jansen MicroscopesLuckily, there was one true Jansen microscope which survived long enough to be studied. As was customary at the time, theJansens made several versions of their new invention to give to royalty. We know that they sent one of their microscopes toPrince Maurice of Orange, and one to Archduke Albert of Austria. While neither of these instruments survived to moderntimes, the later of them was preserved until the early 1600’s, when a Dutch diplomat and childhood friend of Zacharias Jansennamed 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 samemechanical configuration. Drebbel described the royal instrument as being composed of 3 sliding tubes, measuring 18 incheslong when fully extended, and two inches in diameter. It was very ornate, with 3 brass dolphins at the end, forming the feet of atripod. 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 diaphragmsbetween the tubes to cut down on glare from the crude lenses. The microscope at the Middleburg museum was said to have amagnification 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 manymicroscope 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. Butthe microscope wasn’t just a scientific tool. Throughout this century and Victorian times, microscopes were owned by theupper-class as recreational toys.Three Lens SystemAs mentioned before, the first technical advancement of the Microscope after Jansen was revision of it’s optics from the 2-lenssystem 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 theobjective 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, whileavoiding a very large eye-lens from which the eye must be held an uncomfortable distance away. This three-lens system remainsthe 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 until1660 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 everbe useful to make scientific discoveries. Some thought that although it was interesting to look at the tiny legs on a flea or parts ofother insects, you couldn’t really make any new discoveries with such a device. It wasn’t appreciated that by looking at thingsup close, you would be able to see anything really different than at a distance. This thought can be understood by consideringexamination of “milk” under a microscope: nothing really new becomes visible—there just isn’t any structure which becomesapparent at the magnification involved. These early thinkers had no idea that a very important structure lies in all living tissuesthat was within the reach of light microscopes: The Cells.Marcello Malpighi, circa 1660Marcello Malpighi was one of the first great microscopists, and even today is considered the father of embryology and earlyhistology. 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 werewrong, and that blood actually travels in a circular motion from the heart, around the body, then back to the heart. [Since thetime of Galen, it was thought that the blood is produced from the intestines, travels to the liver, then the heart, and then isdistributed to the body by both veins and arteries, where it is consumed.] In this time such a radical idea should have been metwith banishment, yet Harvey’s important position as a top physician to royalty, as well as his meticulous theoretical and logicalproof, made the world stop to consider it. But, as convincing as his argument and evidence was, his theory required some formof 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 thinblood 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 standardequipment 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-helditems, maybe 5-8 inches tall – quite a contrast to the 2-foot tall instruments being made in England around the same period. Theleft-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 hispersonal papers do mention purchase of a Divini microscope, and the item on the right is often considered to be the type usedby 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 isinteresting to note that Italian microscopes remained of this smaller, simpler type for some time, while the English microscopemakers were rapidly making two foot tall monsters with many mechanical innovations.Robert Hooke’s Micrographia, 1665Robert Hooke was a mechanical genius who graduated from Oxford, and worked with Robert Boyle in his famous gasexperiments. Robert Hooke’s Micrographia was an important milestone in proving the importance of microscopy. Thefollowing 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 onwater, and it was firm, yet could compress under force. When Hooke looked at a thin slice of cork under the microscope, itbecame clear why cork had these properties. He could see that the substance was mostly air, with pieces of material making upa mesh-work of supporting structure around the tiny air pockets. Hooke named these pockets of air “cells” after the smallmonastery rooms they reminded him off. While he didn’t at the time understand that these air “cells” were the shadow remainsof what is now considered a cell, the name remains.Hooke’s Microscope by Cock, 1665The microscope used by Hooke was illustrated in his paper. Hooke did not make his own microscopes; they were made byLondon instrument maker Christopher Cock, whom Hooke gave much advice on design. In return, the success of Hooke’sbook 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 fromabove. The shown illumination apparatus, an oil flame and a globe and lens to focus the light, has been said to be made byHooke 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 agreat deal of wobble associated with screwing the focus up and down. Later models made by Christopher Cock had a muchwider 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 SocietyCollection.” These are usually very beautiful instruments, using bright and ornate tooling along the leather tube covers, brassuprights, and Lignum Vitae, a very hard black wood.John Yarwell and the English TripodsLater in the 17th century, microscopes made a morphological change. Probably due to the instability of the thin side-pillar andsmall screw-nose holding the huge tube on the Cock/Hooke style of microscope, the English Tripod became popular.Tripod Sketched an 1631 JournalThe 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. Itis similar in size, body tube, and focus to the side-pillar model of Cock, and the object lies flat on a small platform. Theimprovement was the more sturdy tripod holding the large tube straight up and down.Another 17th Century Tripod, 1680’sMany English-Tripod-Style microscopes were made in the 1680’s, but very few exist today, confined mostly to museums. Theabove 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 ofhis own design, this Dutch Draper/amateur scientist, started making incredible discoveries. He began writing letters to the RoyalSociety in London in 1673, which were published in Philosophical Transactions. He described experiments which could beperformed with simple microscopes, and made exciting drawings of his microscopic world. Leeuwenhoek make the firstdescriptions of protozoa, bacteria, and spermatozoa which he called “animalicules” and made the first detailed descriptions ofthe red blood cell.It would seem incredible that an amateur could out-do the scientific community using simple instruments of his own making, butthe reasons Leeuwenhoek was successful are simple. The lenses in common use in the 17th century were quite crude, oftenmade by smashing molten glass between pieces of wood. Because of optical problems such as chromatic and sphericalaberration (described later) the images at modest magnifications (over 40 or 50X) were blurry, with colorful halos. Whenmultiple lenses are used, this error is synergistically increased. By using only a single, high-power, quality lens, Leeuwenhoekfound 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, withresolutions as good as 2 microns. Until around 1800, the compound microscopes could only resolve as well as around 5microns.A Leeuwenhoek MicroscopeThis 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 aneedle 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 heldvery 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 greatpatience. But for the price of convenience, the images obtainable were greatly superior to the compound microscopes. RobertHooke, 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 weakenedthe sight, which was the reason why I omitted to make use of them though in truth they do make the object moreclear and distinct, and magnify as much as the double [compound] microscopes; nay, to those whose eyes canwell 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 doneon the few remaining copies of Leeuwenhoek’s microscopes, and it appears that some of the lenses may have been made bygrinding, while the best ones were blown. Leeuwenhoek learned that when a glass bulb is blown, a small drop of thickenedglass forms at the bottom of the bulb (much like a drop sits in the bottom of a blown soap bubble.) By carefully breaking awaythe 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 silvermicroscopes were given to the Royal Society of London. Of this very large number, only nine microscopes are known to existtoday. Even the 26 silver instruments which became part of a large and famous collection somehow disappeared. The nineLeeuwenhoek 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 likelycontinue to surface.The Simple Microscope Remains Important:After Leeuwenhoek’s incredible successes, simple microscopes regained their place as complements to compoundmicroscopes. In fact, up until the early 19th century, some of the best microscopes could be used both as simple or compoundmicroscopes. When George Adams made a presentation microscope for King George III at the end of the 18th century, it wasactually 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 smoothfocus, 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 thatdiscovery during this century.John Marshall’s Microscope of 1704By 1690, the two leading microscope makers were John Yarwell (discussed in chapter 3) and John Marshall. Both makerswere trying to capitalize on Malpighi’s discovery of capillaries in 1660, by making new models and advertising them forobserving the circulation of blood through the capillaries in fish tails. Marshall scored a great advantage by having hismicroscope advertised in the very first technical dictionary, Harris’s Lexicon Technicum which was published in 1704. Theadvertisement is pictured above. The book became an incredible success. This is another example of successful bookpopularizing 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 sothat 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 throughthe capillaries can be viewed. The later invention of a sub-stage mirror would have eased this.Another Marshall Style ScopeHere is another Marshall model. Note the focus and positioning options: the body tube can be slid up and down the pillar byloosening a screw, and the stage can be raised and lowered. Fine focusing is by a screw to the side of the pillar which willslowly raise and lower the tube. There were 3 stage options: a stage for opaque objects, which had a bulls-eye lens to focuslight 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 sellingrecently for around $30,000 U.S.The Culpeper Tripod, circa 1725The 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 amirror on a gimble was placed below the hole. This allowed ease of illumination using available light, when examiningtransparent 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, thestraight brass tripod legs were replaced by S-shaped legs (this modification attributed to John Adams Snr., a maker describedlater.) Next, the composite body tube was replaced by brass body tube on a wooden base, then a rack and pinion focusmechanism was added in the early 19th century, and then wood base was replaced with a brass plate in the later versions. Theabove example is a probably circa 1830-50, in the standard pyramid case.Benjamin Martin’s “Drum” Style, 1738Another new design by Benjamin Martin of London was introduced in 1738. This was a small simple microscope where themicroscope body tube is enclosed by an outer tube. There were cut-outs in this outer tube where the object could be placed onthe 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” (reflectingreferred to the light reflecting up from opaque objects.) He made the scopes out of brass, lignum vitae (a hard black wood) andrayskin. Later versions around 1760’s had small angled mirrors below the stage to allow viewing of transparent objects as wellas 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 verycommon, inexpensive style, selling in great numbers. These newer microscopes were larger, made entirely of brass, had eitherrack and pinion or slide focus, and had an adjustable mirror below the stage. Some of the high-quality microscope makers inParis, such as Oberhauser, adopted the drum style for some of their highest quality models in the 1840’s and 1850’s. Late in the19th century, and well into the 20th century, many french makers were producing small and cheap drum-style microscopes astoys and for amateurs. These were available in catalogues such as “Sears Roebuck and Co.” as late as the 1920’s, and areoften 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. Thismicroscope had a much smaller body tube than earlier styles, which allowed more stability. A strong side-pillar holds the tubeand stage firm, with easy access to the stage from all sides (unlike the Culpeper.) Focus is by a fine screw which slides thebody tube up and down smoothly. The stage is cut away in shape allowing more flexibility of accessories and view of awkwardobjects.The real reason this model stood out from the competition of the time was publicity from a popular book – just likeMicrographia did for Cooke’s scope, and Lexicon Technicum did for Marshall’s scope. In this case, Henry Baker publisheda book entitled The Microscope Made Easy in 1742. The book devoted an entire chapter to John Cuff’s inventory of availablemicroscopes. The book was a great success, quickly selling out its first edition, and going through several printings. The Englishinstrument 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 hiscompetition’s book.In 1746, just 4 years after Henry Baker published The Microscope Made Easy George Adams wrote a competing bookMicrographia 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 newmodels, 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 (thereason 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 severalobjective 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 ahuge, 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 OpticImprovements:Before continuing the story of the morphological evolution of the microscope, it’s important to pause and discuss early opticallimitations and how they were overcome.While the 18th century produced some great mechanical improvements for the microscope, making it much more sturdy andeasy to use, the images obtainable remained rather blurry with colorful halos around objects. This was largely due to theproblems of “Chromatic and Aspheric Aberration.” The reason the single lens “simple” microscopes remained importantthroughout the century was that a single lens system has much less aberration because the distortion becomes synergistic withmultiple lenses. This allowed simple microscopes to attain around 2 micron resolution, while the best compound microscopeswere limited to around 5 microns.The Problem of Chromatic AberrationThe problem of chromatic aberration is shown above. Any substance which can bend light (such as the glass in a lens) will bendlight of different wavelengths (colors) slightly different amounts. This leads to the fact that any simple lens will have slightlydifferent focal distances for each color. If an object is white (composed of all colors) as shown above, the red component willcome into focus in a different place from the blue component. As a result, when we focus on a object, it will have a blurry redor blue halo around it.The Secret Invention of the Achromatic LensThe 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 colorsback 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 heknew he had to keep his idea secret until he tried it out. Therefore, he contracted with two different optical shops to make thecrown glass lens and the flint glass lens for him, so they wouldn’t suspect what he was up to. Unfortunately for him, both lensmakers subcontracted the job out to the same lens maker, George Bass, who put 1 and 1 together, and realized what Hall wastrying 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, anothertelescope maker who was experimenting on reducing chromatic aberration. [John Dolland was the father of Peter Dolland, whobecame an important microscope maker later in the century.] After hearing from Bass about the two lenses, Dollandunderstood that this was the answer, and by 1759, Dolland succeeded in making an achromatic lens. What’s more, he took outa 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 achromatictelescope they produced. When word got out that Dolland’s invention was actually invented 20 years earlier, they disputed thepatent rights. The courts decided that even though Hall may have made the discovery, a secret discovery would do the worldno 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 samediscovery 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 muchharder to make in an achromatic style. Because of these difficulties, the first practical achromatic microscope objectives weren’tavailable until around 1800, and it was even later before they were commonly available.Spherical AberrationAfter the chromatic aberration problem was solved, there was still the problem of spherical aberration. This is a distortioncaused because light from the object which hits the edge of the lens does not have precisely the same focal distance as lightwhich comes through the center of the lens. The problem can be made better by using very small apertures or diaphragms tolimit light angles (which we learn later from Ernst Abbe reduces resolution) or by making the lenses with less curvature (whichlimits our ability to make higher power lenses.) If they used multiple lenses to increase magnification power, the error in thedifferent lenses becomes synergistic, and the image gets worse.Joseph Jackson Lister Solves the Problem in 1830The problem was finally solved in 1830, by Joseph Jackson Lister (father of Lord Joseph Lister, the surgeon who discoveredantiseptic technique.) In a paper published in the Philosophical Transactions in 1830, he showed mathematically how ifmultiple low-power lenses are placed a certain precise distance away from each other, you will still get the spheric aberration ofthe first lens, but you will not get any added component from the following lenses. Since the amount of aberration in the firstlower-power lens could be made minimal, the aberration of the whole series would be very small, even though magnificationpower 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 Londonmakers to produce the new objectives. After several years without anyone making his improved objective style, he decided hewas going to have to build a scope himself.A Tulley/Lister Corrected Lens Microscope, 1830’sLister worked with Tulley and Sons to produce this microscope in the mid 1830’s. The additional supports in the back werenecessary 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. Forstability, 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 specificallyfor the new Lister scopes.Adjustable Objective by Ross, circa 1840It’s interesting to note that the Lister-corrected objectives needed extremely tight tolerance of lens position for proper imagecorrection. In fact, the presence or lack of a coverslip, or even different thicknesses of coverslips, would throw off the preciseoptical correction, and degrade the image. Because of this, early Lister-style objectives usually had a slide or rotatingadjustment on them which would need to be placed in one position if no coverslip was used, and the other position if acoverslip was used. An example of these objectives is above, where rotation of the objective would slide the optics to differentalignments. In the mid 19th century it wasn’t even possible to manufacture coverslips to a standard thickness, so it becamenecessary 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 LensesBesides using simple (one-lens) microscopes, there was another way to eliminate the achromatic problem in lenses. Asreflecting 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” lighthitting 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 bluelight – 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, justafter making the reflecting telescope. He was soon joined by Chevalier of Paris, John Cuthbert of London, and many others. Inprinciple, reflecting microscopes should be free of chromatic aberrations, but they are very difficult to fabricate and use. Sincethis was a period of rapid improvement of lenses, the style didn’t compete very well with lenses past around 1840.Rapid Empiric Progress 1820-1850With 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 theymade a new style. They found a number of objects in nature, such as diatoms, which have repeating structures near theresolution of their objectives, and so by looking at these objects with different objectives, it became clear when one had slightlybetter 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 morefactor involved with making a microscope as good as physically possible: angular aperture. While it wouldn’t become provenand well understood until Ernst Abbe publishes a paper in 1877, physical laws dictate that the minimum resolving distance (d) isrelated to the wavelength of light (lambda) divided by a number known as the Numeric Aperture, which is proportional to theangle 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 aslarge of a cone of light as possible from the object.Empiric Increase in ResolutionEven 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 the1850’s, the major makers had reached the theoretical limit of resolution without an immersion (oil) objective. Hence, by thistime, microscopes could distinguish two points as different points if they were at least 0.28 microns away from each other. Ineffect, what they were doing was maximizing the angle of the light from the object to the objective, but they didn’t necessarilyrealize why this was helping.It was in the 1870’s, when experimenting with water immersion objectives (which increase the n in the above equation), ErnstAbbe, working for the German maker Carl Zeiss, elucidated the formula for which he is famous (and brought Zeiss to theforefront in microscope technology.)The Theoretical Maximum is Finally ReachedBy the 1880’s, using oil immersion objectives, a Numeric Aperture (N.A.) of 1.4 had finally been reached, allowing lightmicroscopes to resolve two points distanced only 0.2 microns apart. With the exception of some very unusual immersion fluidsor ultraviolet light, this remains the limit today.Chapter 6.The Microscopes of the 19th CenturyThe nineteenth century was a great time for the microscope. Microscope makers were finally working on the quality of theoptical image, instead of just striving to make mechanical improvements as they had throughout the 18th century. The opticalproblems of the past were corrected through the work of Dolland and Lister by the 1830’s, so that finally large clear imageswere 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) soonafterward.A wide selection of microscopes were being made by the 1850’s. The leaders in quality were the three English firms of AndrewRoss, Powell & Lealand, and R. & J. Beck. Other English firms such as Swift & Son and Watson & Sons became veryimportant in the later century. Cary was making inexpensive, and portable naturalist models. The Drum-style was reborn. TheFrench firms Chevalier, Oberhauser, and Nachet became well known. Later in the century, the German firms of Carl Zeiss andLeitz became an important supplier.This was also the century that America began to make microscopes. This started with Charles Spencer in 1840, who wasknown to make objectives at least as good as the English firms, at a more reasonable price.The English FirmsAndrew Ross’s Lister-Limb, 1837.Andrew Ross made history by being the first major maker to produce a microscope according to Lister’s specifications whichreduce spherical aberration. He was making other microscopes before that, but it was his association with Lister from1837-1841 which made him a leader. Smith (below) and Tulley (last chapter) also made early microscopes in association withLister, but neither of these companies made any significant quantity.Ross Style Introduced 1843This is Ross’s most famous stand, introduced in 1843 which has a triangular bar-shaped limb, and a Y-shaped foot. This stylewas copied extensively by most of the microscope makers for the rest of the century.Smith & Beck’s Style early 1840’sJames Smith produced an early instrument for J.J. Lister in 1826, and he went into partnership with Lister’s nephew RichardBeck, in 1847. In 1851 Joseph Beck joined his brother’s firm, and then Smith retired in 1865. This company (named insuccession “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 1843Hugh Powell made microscopes under his own name in 1840, then went into business with Peter Lealand in 1842. Their mostfamous stand which formed the backbone of their business for over 50 years is shown above. This microscope came in severaldifferent sizes, “No. 1” was the largest, and “No. 4” was a small portable. The triangular bar-limb was copied from Ross, butthe tripod-legs were completely new. Many people considered the Powell and Lealand instruments to be the best in the worldin 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, makingachromatic objectives which were said to be at least as good as the English makers. Above is one of his large Trunnionmicroscopes from around 1855. This had a number of mechanical improvements. The company was slowly joined by manyother 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). SpencerLens 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 andaperture, exploited the use of immersion lenses, and developed “apochromatic” lenses which reduced chromatic aberrationeven more.This was the era of the “Continental Style” frame which became the dominant style at the later part of the century. This style hasthe horse-shoe shaped base, and a fine-focus control on the back of the arm. Most makers were using this style well into the20th century.Another German maker, “Leitz” began delivering quality microscopes around the same time as Zeiss, and survived the hardtimes that put the majority of makers out of business at the turn of the century. In the early 20th century, Leitz was selling agreat number of microcopes annually, and was probably the major source of microscopes during this time.Movement from Brass to IronThe above photo also shows the movement away from all-brass instruments. Brass is an expensive metal, and it is very timeconsuming to polish it to a high-shine and then lacquer it. Later in this century, as competition made cost a more importantconcern, microscope makers started to paint much of the brass paint black rather than polish and lacquer it. Later, they beganmaking the black areas out of iron. By the 20th century, it became common for most of the microscope to be made of blackiron frames, with only the slides, tubes, and mechanical parts made of brass. Even later, it became common to nickel plate theremaining brass pieces, which protected them against corrosion better than lacquer, and was less expensive than the complexlacquering process.Other Styles Common in the 19th CenturyGould 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 around bracket on top of the case where the instrument assembles. Many different makers copied this design, but the onessigned “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 inEngland, peaking in the 1820’s to 1860’s. These were usually unsigned, being made by many makers for the trade. They wereusually inexpensive, lower quality models for students and hobbiests. In France, some of the high-quality instrument makers inthe 1840’s and 1850’s, such as Oberhauser, adopted the drum-style and outfitted it with top quality optics and accessories. Bythe late 19th century, many French makers were making small unsigned models or low quality as toys and for hobbiests. Thesecontinued to be sold, in two basic styles (single pillar from stage to body tube or dual struts) well into the 1920’s, and turn upoften at antique shops.Summary:By the end of the 19th century, the beauty and craftsmanship associated with the microscope was being replaced byhigh-volume, low-cost, mass production. There remains interest in collecting the later microscopes, but their black and chromefeatures 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 suchas 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.