Royal Microscopical Society. 
229 
uncovered point 900, at covered 1100 diameters, at fifty inches from 
micrometer to screen. 
Yet these figures would not give the true relations between the 
magnifying powers of the two objectives as actually used ; for that 
of Mr. Tolies will work through thicker covers than that of Powell 
and Lealand, and for any given cover works nearer the open point, 
so that its magnifying power on covered objects is less than might 
be anticipated from a comparison of the open points of the two 
objectives. 
I have next to state that, Mr. Tolies having been kind enough 
to send me during the past year a number of objectives ranging 
from T Vth to ^Vth for study, I have since February of the present 
year satisfied myself that several of them resolved the 19th band 
of the plate in a very satisfactory manner. These objectives 
differed from each other in many particulars, not merely in con- 
struction, as to which I know nothing, but in performance. My 
examination of these objectives has satisfied me that a high angle, 
with its accompanying advantage of separating power, can be used 
with less detriment to working distance, penetration, flatness of 
field, and other advantages usually supposed to pertain to low 
angle, than has hitherto been supposed, provided the glass is left 
much under-corrected for colour. 
I observed that those of these glasses which were quite under- 
corrected as to colour, not merely gave the best photographs, but 
did the best work by lamplight. This result I may say corresponds 
with what I have been able to observe also with certain objectives 
of Powell and Lealand, Hartnack, and Gundlach. I do not claim 
this observation as a new one, for I have reason to believe that the 
fact has long been well understood in various quarters, but if I am 
not misinformed the average purchaser of objectives demands 
approximate achromatism above all else. Now, in view of the 
irrationality of dispersion, absolute achromatism is impossible, and 
in aiming to approach it as closely as may be the corrections for 
spherical aberration are inevitably sacrificed. It appears important 
that this fact should be more generally known, and I have devised 
the following simple test, which may serve to illustrate the 
subject : — 
If the microscope be placed horizontally on the window-sill of 
a dark room, and illuminated by a solar mirror or keliostat, placing 
a plano-convex lens of about two inches in diameter and ten inches 
focal length so far behind the achromatic condenser that the rays 
cross in the principal focus just before they reach the bottom lens 
of the condenser, it will be found possible with any glass of T Vth of 
an inch focal length or less, combined with suitable eye-piece and 
distance, to project images on a white cardboard screen with powers 
of from 1000 to 10,000 diameters. 
