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XXV. On a Searcher for Aplanatic Images applied to Microscopes, and its effects in 
increasing Power and improving Definition. Pg G. W. Boyston-Pigott, M.A., 
M.D. Cantab., M.B.C.P., F.C.P.S., F.P.A.S . , formerly Fellow of St. Peters 
College, Cambridge. Communicated by Professor Stokes, Sec. P.S. 
Received March 31, — Read April 28, 1870. 
In the observations which I have the honour to submit to the Boyal Society, I purpose 
at present* to describe as briefly as possible — 
I. Some experiments which suggested an inquiry into a method of raising microscopic 
power consistent with a corresponding improvement in the precision of definition, so 
generally destroyed by excessive amplification. 
II. I next purpose to give some account of the inquiries by which the construction 
of an aplanatic-image searcher was gradually arrived at; the object of which was to 
search for aplanatic foci, to compensate residuary errors by new spherical and chro- 
matic corrections whilst amplifying power, and to increase the small interval existing 
between a deep objective and its object, whilst the focal perspective or depth was also 
increased. 
Such an inquiry, — in the present elaborated delicacy of adjustment accomplished in 
microscopes of the highest quality, especially when armed with “ immersion sixteenths ” 
which have alone succeeded in resolving Nobeet’s most delicate bands, embracing lines 
112000 to the English inch, — would seem either superfluous or futile. 
The research was originally suggested by the accidental resolution of the Podura scale. 
This exquisite object, so justly prized by the optician for the trial of microscopes, affords 
peculiar markings resembling notes of admiration, of sufficient delicacy to put even the 
defining-power of objectives of one-fiftieth of an inch to a severe ordeal. I had observed 
these markings to disappear and be resolved into black beads. The objective employed 
had nearly one-seventli of an inch focal length, and an aperture of 50°. The object was 
illuminated by solar rays reflected obliquely by a plane mirror. Having related this 
effect to eminent opticians, I was informed that no objectives (at that time 1862) could 
resolve this test. I prevailed on them, however, to construct a “very fine” one-eighth 
* The writer may here perhaps he allowed to offer an explanation of the delay in presenting this Memoir, 
notwithstanding the substance had been verbally communicated to Professor Stokes so early as the summer 
session of 1869. 
It had been intended to give a much more extended account of the results obtained, and for this purpose, 
during the following autumn and winter, an extended paper was being prepared. In 1870 further delay seemed 
undesirable, and accordingly a new and brief memoir was drawn up of which the present paper forms a portion. 
MDCCCLXX. 4 M 
