|
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
|
