The Aperture Wars
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The Aperture Wars
Research and notes by Jim Solliday


In 1871, a public debate began between Tolles and Wenham over the question of the aperture of immersion objectives.  The debate actually began with a disagreement between Dr. Pigott and Wenham, drawing Tolles in with a challenge put out by Wenham.  The following quote is by Mr. Wenham, "I challenge Dr. Pigott, or anyone, to get, through the object-glass with the immersion front, a greater angle, or any portion of the extraneous rays that would in the other case be totally reflected, as no object-glass can collect image-forming rays beyond this limit" (The Monthly Microscopical Journal, V, 1871, pp.118).  Much of the early debate centered on a 1/10th and a 1/5th made by Tolles.  Eventually, Tolles sent a 1/6th inch lens to Mr. Frank Crisp having an extra front lens intended for immersion (Mayall, Cantor Lectures (1886-88), pp.1119).  This famous lens was the subject of a great deal of correspondence lasting from 1874-1878.  In 1878 Professor Keith computed the angular aperture of the 1/6th finding it to be of 110 degrees (RMS, 1878, pp.142).  This evidence finally settled the debate and completely vindicated Mr. Tolles.  In America the debate was known as "the war of the apertures."  Throughout the debate, Mr. Tolles was assisted by Dr. J.J. Woodward of Washington D.C., and by Prof. Keith of Georgetown (H. Smith, pref.) (Also see, RMS, June 1964, pp.120).


A few events in the productive life of Tolles may help add reference to the development of the homogeneous objective.  In August of 1873, Robert Tolles made a 1/10th inch objective for homogeneous immersion in Balsam, having an aperture of 110 degrees or 1.25 N.A. (Three American Microscope Builders, pp.38)(American Monthly Microscopical Journal, 1884, pp.168) and being a three system lens.  This represents the first true homogeneous immersion system for the microscope (Bradbury, 1969, pp.245) (LM/36/39) (Journal of the Quekett Microscopical Club, V.32, pp.294).  The same month he made his first lens of the duplex front formula, a 1/5th glycerin immersion of 110° balsam angle (n.a. 1.27) (TAMB/38).  Both passed into the possession of the Army Medical Museum at Washington, both were intended for use in softened balsam (AMMJ, 1884, pp.168).  In 1874 his formula and objective were sent to England for inspection and submission in the Journal of the RMS.  This was partly in response to Wenham's denial of the ability of immersion to affect an angular aperture greater then 180° in air (RMS, June, 1964/120).


In 1874, Tolles requested Dr. J. Edward Smith of Cleveland, to report on a 1/6th duplex of 180°.  Dr. Smith confirmed the new lenses excelled all others including higher magnifications such as the 1/15th.  He then received a Tolles 1/10th "duplex" which was judged even better.  Latter that year (1874) Dr. Smith received a 1/10th from Mr. Herbert Spencer (different formula then the Tolles), "the performance of which is not excelled by any glass yet made, be it a fifth or a fiftieth".  There is some doubt if Dr. Smith was using the Tolles in balsam as intended.  Tolles was to have added an emulsion agent to Canada Balsam that inhibited its normal drying time. 


Also in 1873, Tolles designed an instrument for measuring the aperture of his objectives.  He described it as "An apparatus for obtaining the balsam angle of any objective" (The Monthly Microscopical Journal, 1873, pp.212).  In 1874, Mr. R. Keith published in the Monthly Microscopical Journal, proof of the expanded aperture of the Tolles 1/10" objective.  Tolles had described his 1/10th in Nov. 1873 along with its aperture.  The significance of this debate was the fact that an immersion objective could have a greater aperture than corresponds to the maximum possible for a dry lens.  With Tolles objectives the front lens was immersed in balsam having a similar refractive index as the front element, thus establishing a homogeneous system (Mayall, pp.1119).  Tolles distinctly understood the influence of the use of higher refractive media.  By performing tests, Dr. J.J. Woodward confirmed that "the definition unmistakably better than with water immersion" (MMJ, XII, 1874, pp.127).  Both Professor Keith and Dr. J.J. Woodward gave a detailed computation of ray paths through the Tolles balsam immersion 1/10th inch (RMS, June, 1964, pp.120).  The Zeiss oil-immersion lenses were not issued until 1878 (Mayall).  As for who first developed the "homogeneous-immersion formula" with the intention of increasing aperture, Mayall says, "Mr. R.B. Tolles stands alone as the inventor" (Mayall, Cantor Lectures (1886-88), Royal Society of Arts.). 


As mentioned above, in addition to Dr. J. Edward Smith inspecting the Tolles 1/6th immersion, it was sent to England in 1874 and used to persuade the Society in the debate with Wenham over the limits of aperture.  The objective was owned by Frank Crisp and mentioned in the RMS Journal of 1874.  In 1878 Professor Keith computed the angular aperture of the 1/6th finding it to be of 110 degrees (RMS, 1878, pp.142).  This evidence along with the endorsement of Prof. Stokes (an English physicist) finally settled the debate (Three American Microscope Builders, pub. by American Optical Company (1945).


In 1879, John Mayall, wrote that he tested the best objectives available.  He compared the Tolles 1/4th and 1/8th against the best lenses "by English, French, and German opticians," including immersion objectives by the most renowned makers in Europe.  He concluded that, "without reserve of any kind, I say these lenses (Tolles) are the finest I have ever seen."  It was also said at the time, "I find, on the most severe test, there is in Tolles' lenses a better correction for spherical aberration" (How to See with The Microscope; 1885, edition, by J. Edwards Smith, pp.11).  The reputation of Tolles was by now so respected that it let Dr. J. Edward Smith to say of Tolles, "who with almost superhuman genius and energy grapples with the very laws of optics, and bends them to his inflexible will" (H. Smith, 1880). 


Robert Tolles died on Nov 17, 1883 in Massachusetts hospital, Boston.  After his death, Charles X. Dalton took charge of Boston Opt. Works (1883-95).  He was known to continue making microscopes but with his signatures hidden.  Dalton was a workman for Tolles doing his brass work (RMS, June, 1964, pp.124).


Even though Robert Tolles prevailed in the Aperture Wars, his rival Wenham was no slouch.  It is only fair that a few good words be said of him and his considerable contributions to microscopy.  Francis Wenham lived between the years of 1824-1908; he was born in Kensington, London.  His greatest contribution to England was his influence on the development of the steam engine.  However, in microscopy he was known to have made many of his own objectives as well as for others.  He developed a number of binocular prism designs beginning in 1853 to his most common and well know in 1860.  By 1855 Wenham suggested that the correction collar act on the rear elements of the objective thus perfecting the system and being the form that survives to this day.  Until then the Ross system of moving the front element was used.


In 1850 Wenham rediscovered Goring's idea of using a single front element in British objectives.  French opticians had been acting on the idea for sometime.  It was originally Dr. Goring who realized that over-correction was a problem and suggested that the first element be a single lens instead of the usual compound element.  This would allow the excess correction of the back combination to balance the whole.  Most British firms continued making front doublets for some time. 


In 1851, Wenham designed his metal paraboloid for dark-ground illumination (first form) (LM/245).  L. Martin uses the year of 1850 for Wenham's Paraboloid design.  Wenham’s paper on the subject was published in the Transactions in 1852; however, Smith & Beck manufactured one for the Great Exhibition in 1851 (JQMC, V.32, pp.290).


In 1853, Wenham presented his first paper "On the Application of Binocular Vision to the Microscope"(Trans. Micro. Soc, Lond. 1-13, 1854).  Smith & Beck constructed the first stand using this system.  This was the beginning of a number of designs using a refraction achromatic bi-prism, the last of which came in 1860.  This final form consisted of a rhomboid prism, which reflected the rays from half the objective aperture into a side tube.  This arrangement was known as the Wenham binocular and lasted for over a half century.  Certain British makers continued to make the system until 1939 (LM, Hartley, 1991). 


In 1870, following the death of Thomas Ross, Francis Wenham was persuaded to join the firm as adviser (Ross & Co.) (1870-1882).  He redesigned the Ross bar-limb with the Jackson Limb and eventually used the Zentmayer horizontal lever with its second slide as the fine adjustment (Ross Zentmayer).  Wenham also designed the famous Ross Radial Microscope of 1882 (LM/36).  Earlier in 1875 he had written an article on the benefits of oblique illumination with high power objectives.  In 1877, Wenham took out patent No.496 for an “Object stage for Microscopes” designed to facilitate oblique lighting, a preoccupation which led to his new radial design of 1882. 


At this point in his life and after the humiliation of the aperture war it was said that he gave up microscopy and never talked to any member of the Microscopical Society again. 


J. Solliday




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