Charles A. Spencer 

and His Early Microscope Objectives

Deborah Jean Warner

National Museum of American History

Smithsonian Institution

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Evidence is presented from a variety of users that attests to the quality of the optical work of Charles Spencer. His microscope objectives compared very favorably and often exceeded the resolution of examples made by those considered the premier makers in Europe. Unfortunately and despite Spencer’s obvious optical knowledge and skill, he did not divulge the details either in print or to his contemporaries.



Charles Achilles Spencer (1813-1881) is well-known as having been America’s first notable microscope maker.1  But, in the words of a well-informed contemporary, his instruments were “unrivalled for achromatic quality, and power of definition, though miserably deficient in accessories and mounting.”2

Spencer came from Canastota, a mercantile village in Madison County, New York.  He began in business in the mid-1830s, at a time when achromatic microscope lenses were just becoming available, and scientists in several fields were beginning to appreciate microscopic research.  Young American doctors flocked to Paris to learn the techniques of medical microscopy from Charles Robin and other pioneering practitioners.  Benjamin Silliman, professor at Yale and leading member of the American scientific community, announced that geology had derived “powerful aid from the microscope.”3

According to one obituary, Spencer “had none of the traits of a money-maker, and his whole life was troubled by poverty due very largely to his inability to carry on a business by business methods.”  Moreover, he “would suffer” no objectives “to go from his shop until tested by him and found perfect, and to do this he would take time, and that discouraged patrons.”  At times he had orders on hand for $25,000 or more, from customers in the United States and abroad.4

In 1846, apparently confident enough to approach microscopical experts, Spencer went to New York City.  There he met Chandler Robbins Gilman, professor of obstetrics at the College of Physicians and Surgeons, and proud owner of an expensive achromatic microscope made by Charles Chevalier in Paris.  Spencer examined Gilman’s instrument, claimed he could do better, and convinced Gilman to place an order.  In 1848, Gilman sent a glowing account of his new Spencer microscope to the American Journal of Science.  As with the smaller Chevalier, it could be used in the horizontal or vertical position.  But, while Chevalier instruments were designed to mount on a wooden box, the Spencer was supported on a tripod, as were those made by Andrew Pritchard in London.  More to the point, Spencer provided this stand with three powerful objectives: of 1/3, 1/7, and 1/12 inch focus.  A committee of the New York Lyceum of Natural History (now the New York Academy of Sciences) also weighed in on the matter, reporting that the “superiority of Mr. Spencer’s microscope over Chevalier’s No. 1 is very decided, although made on a much smaller and less expensive scale.”5

Jacob Whitman Bailey, professor at the United States Military Academy and foremost American observer of fossil infusoria, described Gilman’s new objectives as “far superior to any of Chevalier’s which I have yet seen” and “a proud triumph for American art.”  Bailey then ordered a Spencer objective for himself and, after testing it on a Navicula hippocampus, a particularly difficult diatom he had recently received from an English colleague, noted that it had “much better defining power” and “a larger angle of aperture” than others he had seen.  He also noted that Spencer had distinguished the striations on a particularly difficult infusorium that he had found in the Croton River, and that he named Navicula spercerii.6

Bailey also urged Spencer to send an objective and eye-piece to Mr. Cole in Boston, “as possibly worthy of the Rumford Medal” given by the American Academy of Arts and Sciences.  The reference here was to Thomas Cole, a teacher in Salem who observed infusoria “as the occupation and amusement of leisure hours, without reference to any scientific purpose.”  At the May 30, 1848 meeting of the American Academy, Cole read a letter in which Spencer detailed “the history of his attempts at constructing achromatic microscopes, and of the improvements he had effected.” But no medal was awarded. 7

When the American Association for the Advancement of Science met in New Haven, Conn., in the summer of 1850, several fine English and American microscopes were on display, and a three-man committee was appointed “to examine them and report thereon.”  There was no mention of Spencer in the published proceedings, but his work must have been on the table.8  In 1851, when the Association met at Albany, there was a report from the committee charged with examining Spencer’s best microscopes and lenses.  The five members (John W. Bailey, John Torrey, J. Lawrence Smith, Waldo Irving Burnett, and Alonzo Clark) agreed that Spencer’s lenses were of “unrivalled excellence.”  And while leery of undue boastfulness, they believed “it would be an act of injustice not to state their sincere conviction that Spencer’s objectives are now the best in the world.”9

Soon thereafter, some microscopists asked Spencer to explain the secrets of his success.  His reply, published in a Boston newspaper and reprinted in the American Journal of Science, is that of a cagey craftsman.  Most optical artists, he said, made microscope objectives by combining three double achromats; the parts of each were of crown glass and dense flint glass of Guinaud, cemented together, and “separately corrected for figure and color, or nearly so.”  But this design got them only so far.  When the artists adopted the ideas that Joseph Jackson Lister had presented to the Royal Society, improvements followed rapidly.  But they “soon again reached a limit, beyond which the artist had no expectation of success.”  Facing this situation, Spencer said, “I undertook a series of experimental investigations in the manufacture of glass for optical purposes,” and “Many new, unexpected, and valuable results were obtained, in reference to both the optical and physical characters of this interesting material.”  Spencer considered the angle of aperture to be the “most essential character” of an objective, and so was exceptionally proud of having produced lenses with apertures of 170°.  And he ridiculed “the first of European artists” who had claimed to have reached the limit of perfection with a 1/12 objective which had an angular aperture of 135°.10

Irving Burnett was a Harvard graduate described as “the first in the United States to attempt a full assimilation of the European literature on microscopy and histology and to devote his energies, thought a brief but active life, to the study of this field.”11  He took a Spencer microscope to Paris in 1850, compared it with some European instruments, and concluded that Spencer “is now a rival at least of the best foreign artists.”  Burnett also noted that the angle of aperture of Spencer objectives “was wider than that of the others, a larger field and more light being of course the result.”  And, “the secret of success with Mr. Spencer is, that he makes his own glass, and after much experimenting, he has been so fortunate as to produce a form capable of being wrought with a very wide angle of aperture, yet with no more than the ordinary liabilities to chromatism and spherical aberration.”12  J. Lawrence Smith, a graduate of the Medical College of South Carolina, also took note of these European trials.  In his words: “We cannot bestow too much praise on our American maker, for the immense progress which he has made in the construction of objective lenses, and it is to be regretted that he has not chosen a better mounting for them, than that of Chevalier, which is very defective and prevents good glasses from showing their best effects.”13

John Bacon, Jr. was a doctor in Boston who, in 1851, reported having observed the dumb-bell urinary deposit, now known as a marker for Bright’s disease, using a “very excellent” Spencer microscope.”  An inventory of Bacon’s microscopical cabinet, compiled after his death in 1881, included six Spencer objectives.14

Alexander Smith Johnson was a grandson of President John Adams and a judge in Utica, New York.  After visiting Spencer in July 1851, he informed the American Journal of Science that he had had the pleasure of seeing a 1/12th inch objective which Spencer “had then lately completed upon his new formula.”  Johnson went on to say that the “qualities and performances” of this lens “so far exceeded those of any objective that I have ever seen or heard of, that I cannot doubt your microscopic readers will be pleased to be informed of them.”  With an angular aperture 174½°, it could resolve Navicula Spencerii and Nobert’s test of 15 bands (a ruled surface produced by Friedrich Adolph Nobert, a watchmaker in Pomerania).  There was apparently a bidding war for this objective, and Johnson won.15

Oliver Wendell Holmes, a professor at the Harvard Medical School, visited Spencer in the summer of 1851, bringing two objectives (a 1/8 and a 1/12) made by Andrew Ross, and belonging to the Lowell Institute in Boston.  Holmes compared these English objectives with two Spencers (a 1/4 and a 1/8) and, as he reported to the American Academy of Arts and Sciences, found the “superiority of Mr. Spencer’s glasses” to be “unquestionable,” especially “on the delicate tests Navicula Spencerii and Grammatophora.”16  Years later, when addressing the Boston Microscopical Society, Holmes recalled having visited Mr. Spencer twice at Canastota, “a small interior town with the burnt stumps of the forest trees all round it, and felt that I was a pilgrim to the microscopic Mecca.”  Holmes also said that Spencer, “wrought lenses that turned the London makers paler than blue glass would make them look, and with angles of aperture that straddled far outside of the limits which Andrew Ross, the king of them all, declared to be the boundaries of the possible.”17

John Leonard Riddell, a professor at the Medical College of New Orleans, asked Spencer “for the finest objective of high power which he could make, expressly without limit as to price.”  Spencer sent the objective in May 1852, with a note saying that it was “the best he had ever made,” and charging the “moderate sum” of $120.  In describing this objective to his colleagues, Riddell noted that “its defining power is so great and so wonderfully accurate, that a sum of money greater than I choose to name, would not deprive me of its possession.  It is rated by Spencer as one-sixteenth of an inch focus, though the available working focal distance is probably less than 1-20th of an inch, requiring the very thinnest of Chance’s thin glass, for covering objects to be seen.  Its angle of aperture is full 174°!  A figure at least forty units beyond what the best European opticians have, until quite lately, considered practicable.”18

Joseph Burnett, a pharmacist with a shop on Tremont Row, in Boston, announced in 1852 that he was “Agent for the sale of Spencer’s microscopes,” and had “just received two instruments from this celebrated maker, which he offers for sale.”19

By 1852, the Cabinet of Philosophical Apparatus at Amherst College boasted a “Compound Achromatic Microscope” made by Spencer.  This was equipped with three objectives (of 1 inch, 1/2 inch, and 1/4 inch focus), three test plates (of Navicula Hippocampus, Navicula Spencerii, and Grammatophora Subtilissima), an animalcule cage, a camera lucida, a micrometer eyepiece, and a polarizing combination.  Edward Hitchcock, the president of Amherst, described this as “one of Spencer’s best Microscopes,” noting that it had been acquired by Ebenezer Strong Snell, the professor of mathematics and natural philosophy at the College.20  This was apparently lost in 1882, when Walker Hall was destroyed by fire.

Hamilton L. Smith was a Yale graduate who taught at Kenyon College in Ohio and edited the periodical, Annals of Science.  He called attention to Spencer in December 1852, referring to him as “a well-known American artist, whose objectives are pronounced by those who have used them superior to those of any other artist.”  Smith also published a letter from Spencer, giving the prices of first class objectives, from 98° to 170°, and costing from $35 to $125.  “The extent of our orders for special objectives for other instruments,” Spencer went on to say, “is already so great as to interfere with the execution of our own complete microscopes.”  By March 1853, Smith had compared the performance of a Spencer 1/4 inch objective with a very fine Oberhauser of the same power and a Nachet of much shorter focus, and could report the result to be “entirely in favor of Spencer.”21

Two Spencer microscopes were shown at the International Industrial Exhibition held in New York in 1853.  The British Commissioners regarded these as the best microscopes in the Exhibition, adding that their chief merit consists “in the perfection of the lenses; the objectives are far superior to any others in the Exhibition, and in their powers of definition and penetration, and in the abundance of light they admit, they are believed to be quite equal to any that have ever been produced; these lenses have been constructed according to formula (worked out by Mr. Spencer himself) which have secured for them, together with every requisite correction, an angle of aperture which is said by competent judges to be much larger than has been attained by any other maker, and one which (in the higher powers at least) is not likely to be surpassed.”22

William Henry Dwinelle was a progressive dentist who recognized the advantages that microscopy offered his profession.  In 1853, Dwinelle noted that on a recent trip to England, he had been surprised to learn that many leading English microscopists questioned the value of lenses of large angle of aperture.  He then called attention to investigations in which Spencer had extended this angle from 135° to 175°.  In a subsequent paper, Dwinelle noted that to the surprise of everyone but himself, Spencer “recently attained an angle of 178° in his 1/12 objectives, thoroughly corrected in all their relations, and of the highest practical availability.”  Dwinelle went on to say that a committee of the American Medical Association had tested Spencer’s objectives against those of foreign make, and decided “that his glasses were quite superior to any now made in the world.”23

Silas Durkee, a graduate of the Bowdoin Medical School (Brunswick, ME) who practiced in Boston, reported in 1854 having taken a substance from the nostril of a patient and, by means of a “Spencer’s microscope,” seen a something he had never seen in pathological specimens.  Searching the literature, he identified this something as a Sarcina Ventriculi as seen by John Goodair in Britain.24  A few years later, Durkee told the Boston Society of Natural History about a new orthoscopic eyepiece he had obtained from Spencer.  And he quoted a letter in which Spencer said that eyepiece was “more perfectly achromatized than the old negative form, has a perfectly flat field, and is more luminous.”  Spencer recognized that the cost was high (fifteen dollars), but was “satisfied it will meet a want long felt by microscopists.”25

Thomas F. Bergin, an inventor and microscopist in Dublin, Ireland, had a Spencer 1/12-inch objective by the mid-1850s.26

Lewis Morris Rutherfurd, a New York lawyer with sufficient wealth to indulge his scientific enthusiasms, visited Giovanni Battista Amici in Florence in early 1850, and sent Spencer a sample of the Lillhaggsion fossil flour that Amici had been using to test his objectives.  Spencer examined the sample with his new 160° (0.98 NA) objective, which he described as displaying rather perfect chromatism.  And he sent Amici a sample of Grammatophora subtillisima Bailey, a test object from Rhode Island that neither he nor others had yet resolved.27  Francis Peyre Porcher, a recent graduate of the Medical College of the State of South Carolina, visited Amici in early 1854, and “saw one of Spencer’s in his room.”  Porcher also noted that Amici praises Spencer’s “ingenuity and skill,” but “declares that his lenses have not the power and clearness of his own.”28

Joseph Henry, the physicist who served as the first Secretary of the Smithsonian in Washington, D.C., purchased wonderful scientific apparatus to show American scientists and educators what could be had and, where possible, to support American artisans.  In 1854, he told the Board of Regents that the achromatic microscope was “an article of apparatus which, within a few years past, has opened almost a new world of research in the phenomena of lift and organization, the use of which is now indispensable in advancing our knowledge of physiology and its kindred branches of science.”  Since the best European microscopes seldom came to the United States, and since those that did come here were enormously expensive, Henry had been interested to learn, “from a source that could be relied on, that an individual in the State of New York had successfully devoted himself to the study of the microscope, and that he was able to produce instrument of this kind which would compete with the best of those constructed in Europe.” Later that year, the Smithsonian paid C.A. Spencer & Co. $381 for apparatus.  This included $100 for the trunnion mount, $20 for a moveable stage, $20 for a polarizing apparatus, $5 for a camera lucida, and more for various objectives.  This instrument, alas, was lost in the fire that swept through the Smithsonian Castle in 1865.29

In March 1856, the University of Michigan named a committee “to contract for the construction of a suitable microscope for the University.” Within a year or so, the two committee members — Abram Sager, a physician then serving as Dean of the Medical School, and Alexander Winchell, a professor of geology who would soon be named Geologist of the State — had spent $469 for a Spencer microscope.  This enormous sum was charged to the account of “Natural History.” Twenty years later, this microscope was transferred to the Physiological Laboratory in the Medical School.  The transfer was arranged by Charles Stowell, a young doctor who would spend his career teaching physiology and microscopy at Michigan, and who was clearly aware of its historic importance.  In an obituary notice that he penned shortly after Spencer’s death in 1881, Stowell explained that the objective was a 1/16 of “as near 180°as can be obtained.”  Then, garbling the history a bit, he claimed that Winchell had exhibited it at the American Association for the Advancement of Science meeting in Albany in 1856, and the assembled experts declared it “the finest objective in this country.”  When Stowell got his hands on this objective, he saw a crack “running across about 1/3 of the field,” and so returned it to the firm.  Spencer replied that he could make a new objective nearly as cheap as he could remedy this, “for it is one of my first glasses.” Accepting the inevitable, Stowell ordered a new 1/18.30  This microscope is now in now in the National Museum of American History.  The objective had not been measured, but does not appear to have a crack.

At a meeting of the Boston Society of Natural History in the summer of 1857, the otherwise unknown Charles K. Stevens, of Lawrence, Mass., “exhibited one of Spencer’s microscopes, to which were adapted a second set of lenses made by Nachet of Paris, and embodying some recent improvements.”31

Louis Agassiz, the remarkable Swiss naturalist who landed a professorship at Harvard and established the Museum of Comparative Zoology, began his microscopical work with instruments made by Joseph Fraunhofer in Munich, and by Georges Oberhaeuser in Paris.  In time, however, he came to appreciate Spencer’s achievements.  The details here come from Henry James Clark, a young naturalist who had learned from and worked with Agassiz at Harvard.  Speaking to the American Academy of Arts and Sciences, in 1859, Clark stated that, by using one of Spencer’s “most recently improved microscopes,” he had been able to “solve the mystery” of glandular dots in white pine.”  In a footnote, Clark explained that Agassiz had seen “the workings of an instrument of the English pattern, made by Spencer” in 1852, “and although it was known as a rival of, if not superior to the Transatlantic microscopes, he did not become convinced that it came up to his requirements.”  In 1857, however, “having heard of Spencer’s continued rivalship and growing superiority,” and having “determined to test his skill to the utmost,”  Agassiz sent Clark to Canastota, to advise Spencer “as to the nature of the work for which we wished to use his instruments.”  Clark spent several days with Spencer, “testing his objectives with the tissues of every creature which we could find.”  And as recounted: “I shall never forget the astonishment and delight with which I occupied day after day, plunged into the hitherto unknown depths of organic life.  I say this after having tested from time to time some of the best English microscopes which have been made since the ‘Great Exhibition.’”  Clark’s report convinced Agassiz “that we must have three sets of [Spencer] objectives; — one with the extremely flat field; a second of the like kind, but so put together as to allow working with it plunged in water; and the third with a depthing focus extending as far as possible beyond that of the ordinary kind, for the purpose of viewing objects as a whole, in order to ascertain the relations of their different parts.”  Returning to Canastota in the summer of 1858, Clark obtained “one of Mr. Spencer’s quarter-inch objectives, with an angular aperture of one hundred and forty-five degrees.”  Clark concluded his account by saying that “Spencer is now devoting those extraordinary abilities which show him to be a man of genius, to the construction of a microscope which shall embody not only the optical excellences of the different systems of lenses required for the various modes of investigation, but also those conveniences of mounting which the long use of that instrument has taught us, to facilitate the researches upon the living being in its normal condition, and in its element, that we may be no longer compelled to represent the tortured figures of a crushed body of dismembered organism.”32

In this same paper, Clark also noted that: “I have for several years past asserted that the next step in the increase of the magnifying powers of the microscope would be accomplished by the construction of a new form of eye-piece, which would augment the image formed by the objective to an almost unlimited extent.  At last I am happy to find my prediction verified, in the most practical manner, by the ‘orthoscopic ocular’ invented by Spencer.  With such a range of powers, then, there is hardly any body of moderately transparency but what may be minutely investigated to its very core.”

Dr. Rufus King Browne was a dental microscopist who visited Canastota in the summer of 1860.  During his stay, Spencer made for him a 1/8 objective that, according Spencer’s son Herbert, “was considered by all who saw it to be the best ever made up to that date.”  The angular aperture was 175°.  One of the systems of this objective contained fluorspar.33

Addressing the Academy of Natural Sciences of Philadelphia in 1904, Frank Keeley described a Spencer objective that was made in 1860, and that had a 1/4 inch focus and an aperture of between 142 to 152 degrees.  Keeley noted that when the back system was taken apart for re-balsaming, “it was found to consist of five lenses, three of which were convex and two concave.”  And, recognizing that one of the lenses was fluorite, he opined “that Spencer abandoned the use of fluorite because he realized that sharp definition was more important than the elimination of last trace of color, rather than from any fear of its lack of permanency.”34

Charles E. West, lawyer and educator in Buffalo and Brooklyn, had a Spencer trunnion microscope, and eight Spencer objectives.35

Samuel Baldwin Ward, a professor at the Women’s Medical College of the New York Infirmary, and president of New York State Medical Society, had Spencer’s horizontal microscope (#12), but was probably not the original owner.  This is now at the New York State Museum at Albany.

According to the 1850 Census of Manufacturing, Spencer had $9000 invested in his business, employed 10 male hands, and produced 100 completed achromatic microscopes (worth $11,780) and 6 oxy-hydrogen microscopes (worth $1000) each year.  Spencer turned his attention to telescopes in the mid-1850s, working with Asahel Eaton to build a large refractor for Hamilton College.  I don’t know what he did during the Civil War.  By the mid-1860s, however, he was trading as C.A. Spencer and Sons, and was producing mid- and low-power objectives suitable for increasingly routine work.

  1.  Oscar W. Richards, “Charles A. Spencer and His Microscopes,” Rittenhouse, 2 (1988): 70-81.  James H. Cassedy, “The Microscope in American Medical Science, 1840-1860,” Isis, 67 (1976): 76-97.  Hamilton L. Smith, “Memoir of Charles A. Spencer,” Proceedings of the American Society of Microscopists, 4 (1882): 49-74.
  2.  Review of William B. Carpenter, The Microscope and Its Revelations, in Buffalo Medical Journal, 12 (1857): 348-350.
  3.  Benjamin Silliman, Address Delivered Before the Association of American Geologists and Naturalists (New York, 1842).
  4.  “Charles A. Spencer,” Scientific American, 45 (1881): 78-279.
  5.  C.R. Gilman, “An Account of a Compound Achromatic Microscope, made by Charles A. Spencer of Canastota, N.Y.,” American Journal of Science, 5 (1848): 237-240.
  6.  J.W. Bailey, “Letter,” in American Journal of Science , 5 (1848): 238-239; “Spencer’s Microscopes,” pp. 285-286; and “Spencer’s Microscopes,” p. 443.
  7.  Bailey to Spencer, June 1848, quoted in Smith, op. cit., ref 1, p. 62.  Thomas Cole, “List of Infusorial Objects Found Chiefly in the Neighborhood of Salem, Massachusetts,” Proceedings of the Essex Institute, 1 (1848-1856): 33-48.  Note in Proceedings, American Academy of Arts and Sciences, 2 (1852): 2.
  8.  “Section of Geology and Natural History,” Proceedings, American Association for the Advancement of Science, 4 (1850): 122.
  9.  “Report of the Committee on Microscopes, on the Lenses Exhibited to the Association by Mr. Spencer,” Proceedings, American Association for the Advancement of Science, 6 (1851): 397-398, and vi.
  10.  “On the Improvements by Mr. Chas. A. Spencer in Microscopic Object Glasses,” American Journal of Science, 13 (1852): 290-292.
  11.  “Waldo Irving Burnett,” Massachusetts Medical Society, Medical Communication (Dec. 1, 1854): 54-56.  Jerome Cornfield, “Waldo Irving Burnett, Early American Histologist,” Bulletin for the History of Medicine, 26 (1952): 430-451.
  12.  W.I. Burnett, “On Microscopes and Microscopy,” American Journal of Science, 11 (1851): 56-63.
  13.  J. Lawrence Smith, “A comparative examination of the objective glasses of Microscopes from Mr. Ross of England, Mr. Spencer of America, and M. Nachez (sic) of Paris,” American Journal of Science, 11 (May 1851): 277-278.
  14.  John Bacon, “Observations on the Dumb-Bell Urinary Deposit,” American Journal of the Medical Sciences (April 1851): 297-306.  “Prof. John Bacon,” American Monthly Microscopical Journal, 2 (1881): 235.
  15.  Alexander S. Johnson, “Notice of a New Object Glass Made by Charles A. Spencer of Canastota, N.Y.,” American Journal of Science, 13 (1852): 31-32.  Hamilton L. Smith, “Memoir of Charles A. Spencer,” Proceedings, American Society of Microscopists (1880): 49-74, on 64.
  16.  O.W. Holmes “On the Use of Direct Light in Microscopic Researches,” Proceedings, American Academy of Arts and Sciences, 2 (1852): 326-332.
  17.  O.W. Holmes, “An Address Delivered at the Annual Meeting of the Boston Microscopical Society,” Boston Medical and Surgical Journal, 96 (1877): 601-612, on 609-610.
  18.  J.L. Riddell, “Spencer’s Objectives for Microscopes,” New Orleans Medical and Surgical Journal, reprinted in Ohio Medical and Surgical Journal (Sept. 1852): 49-50, and in Buffalo Medical Journal, 8 (1852-53): 177.  J.H. Cassedy, “John L. Riddell’s Vibrio biceps.  Two Documents on American Microscopy and Cholera Etiology, 1849-59,” Journal for the History of Medicine, 28 (1973): 101-108.
  19.  Burnett ad in Boston Medical and Surgical Journal, 46 (Feb. 28, 1852): advertising sheet.
  20.  E.S. Snell, “Catalogue of the Articles belonging to the Cabinet of Philosophical Apparatus in Amherst College” (1852).  Edward Hitchcock, Reminiscences of Amherst College (Northampton, 1863), p. 71.
  21.  “Objectives for Microscopes,” Annals of Science, 1 (1853): 43; and “Spencer’s Objectives,” 138-139.
  22.  “Special Report of Mr. Dilke,” in New York Industrial Exhibition, Reports from British Commissioners (1854), see pp. 78-79.
  23.  William Henry Dwinelle, “Microscopes of Large Angle of Aperture,” American Journal of Dental Science, 3 (1853): 236-242.  Dwinelle, “Essay on the Microscope,” American Journal of Dental Science (1855): 126-139.
  24.  Silas Durkee, “Sarcina Ventriculi in connection with Disease, &c.,” American Journal of the Medical Sciences (Jan. 1854): 96-102.
  25.  Notice of letter from C.A. Spencer to Dr. Durkee, in Proceedings of the Boston Society of Natural History, 6 (1859): 246.
  26.  Mentioned in Hamilton Smith, op. cit. (ref. 1), p. 71.
  27.  G. Merico, “Giovanni Battista Amici’s improvements in microscopic optics assessed from his test objects,” Atti della Fondazione Georgio Ronchi63 (2008): 837-864, on 849.
  28.  Francis Peyre Porcher, “Letter to the Editor,” Charleston Medical Journal and Review, 9 (1854): 416-428, on 422.
  29.  Daybook, 1846-1856. Smithsonian Institution Archives. Smithsonian Institution, Annual Report for 1854 (1855), p. 138.
  30.  Wilfred Shaw, ed., The University of Michigan: An Encyclopedic Study (Ann Arbor, 1956), Part V, p. 800.  Charles H. Stowell, “Charles A. Spencer,” The Microscope, 1 (1881): 129-133.
  31.  Notice in Proceedings of the Boston Society of Natural History, 6 (1859): 247.
  32.  H(enry) James Clark, “Some Remarks Upon the Use of the Microscope, as Recently Improved, in the Investigation of the Minute Organization of Living Bodies,” Proceedings of the American Academy of Arts and Sciences (Jan. 26, 1859): 136-149; reprinted in American Journal of Science, 28 (1859): 37-48.  Christoph Irmscher, Louis Agassiz (Boston, 2013), p. 180.
  33.  H.R. Spencer, “Letter,” Proceedings, American Society of Microscopists, 12 (1890): 248-249.
  34.  “A Spencer Objective,” Proceedings, Academy of Natural Sciences of Philadelphia, 56 (1904): 475.
  35.  Charles E. West, “Forty Years’ Acquaintance with the Microscope and Microscopists,” Proceedings of the American Society of Microscopists, 8 (1886): 161-173.