Monty Merc | Royal Microscopical Society. 305 
fine black shadow is impossible from decussating rays. If we want 
to examine any new object perfectly by the unaided sight, the first 
thing we do is to carefully set it in the most favourable light, be it 
gem or picture, bust or insect. Can we be exempted from the same 
pains in getting an «mage of a delicate tissue to be transmitted to 
the eye ? 
When in 1862 I first began to study the Podura spherules, I 
was greatly assisted by first assuming their sphericity, and then 
trying whether the shadow law applied to them. Could a black 
shadow be brought out on any side of Formoswm beads finely 
developed with a ,),th obtained that year (1862)? (The beads 
were readily seen with the 1th Messrs. Powell and Lealand were 
so good as to very skilfully construct for me.) This required a ro- 
tating stage. I found all the crescentic shadows rotated with it: 
Next, could the Podura bead show them similarly ? I found this to 
be a constant result. Sphericity thus seemed demonstrated. Would 
blurred shadows entirely obscure or “blot” out the delicate tracery 
requisite for a fine definition ? In telescopes no such difficulties on a 
clear night are in the way of seeing. A sharp shadow by parallel 
rays surpasses all others, provided diffraction effects can be pre- 
vented in the microscope. Thus the celebrated astronomer Dawes 
could discern, with his remarkable observing powers, a satellite’s 
shadow on Jupiter, with an 8-inch refractor and a power of 100. 
Such a point would have been invisible if the outline were blurred 
off insensibly by a multitude of non-parallel illuminating rays ; or 
even if the satellite were removed a much greater distance from 
Jupiter. Just as a sunbeam will not cause a sharp shadow of a 
small sphere, except the shadow is taken on a screen close to it. 
IT have found fine effects by using a 43-inch objective, with an 
aperture cut off on the front lens to y}oth of an inch, transmitting 
a sunbeam reflected from a plane mirror. With a power of 4000 
and a y'gth, I have seen beading in the most remarkable manner in 
diatoms, &c., when thus illuminated, 
It may be imteresting to some of the readers of this paper to be 
informed that my first paper, dated May 21, 1869, and read in 
December of that year, was to a great extent a transcript of a paper 
written in 1863, before I had any idea of using intermediate lenses, 
The drawings of the crowded beading of Lepidocyrtus eurvicollis, or 
Test Podura, were obtained with a very fine quarter of Andrew Ross, 
marked 1851, an eye-piece, C, and about 4 inches of draw-tube. 
An E eye-piece of Powell’s, with the eye-lens or front glass placed 
in contact with the field glass, and a very fine radial slit for the 
illumination, and direct lamp-light without reflexion, were the means 
employed at that time. The original drawing, anastatically printed; 
has lost its peculiar beauty in .copying; but I still adhere to this 
