ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 
355 
above the objective due to aperture, and not to changes below the 
objective resulting from diffraction by the finer details of an object. 
He first made a preliminary experiment by constructing a gigantic 
Microscope out of a telescope objective, which had a diameter of 2J in. 
and a focus of 43 in., and an eye-piece which was supported 60 ft. 
from the objective. No tube was necessary, as the experiment was done 
in a dark room. The source of light was an electric arc lamp about 
27 ft. from the objective. The object was a series of vertical lines 
scratched with a fine needle-point through the opaque film of an old 
dry plate negative supported nearly 46 in. in front of the objective. 
After focusing so as to show the lines through the eye-piece, a plane 
was found not far from the eye end of the actual telescope-tube, in 
which was a central image of the electric arc with a series of diffraction 
images on each side. These images could be dealt with so as to vary the 
final image seen through the eye-piece, as one deals with the “ spectra ’* 
at the back of the Microscope objective to produce change in the final 
image of the ordinary Microscope. 
If a telescope objective behaves in the same way as a microscopic 
objective in experimenting with these diffraction phenomena and asso- 
ciated image changes, does a reason based on such experiments exist for 
regarding microscopic vision as sui generis ? 
Not only have microscopists noticed in practice the direct relation of 
aperture to resolution, but also the fact that isolated lines or particles 
in an object appear broader through an objective of small aperture, and 
narrower through an objective of large aperture. This narrowing effect 
of increasing aperture is due to the contraction of the diffraction pattern. 
It is easily understood that the projected image discs of a series of close 
points in an object (or the projected image bands of a series of close lines ) 
might touch or overlap when projected by a lens of small aperture, and, on 
the other hand, might be separated or resolved when projected by a lens of 
sufficiently large aperture. 
The separating or resolving power of the telescope is thus ex- 
plained. 
Simple parallel experiments with the telescope and Microscope show 
that the actual effects of aperture in both instruments are in harmony 
with the above explanation. 
Dr. Mercer now describes 14 experiments, of which the following 
seem the most important. 
Experiment 1. — The instrument used was a telescope having an aper- 
ture of 2J in. and a focus of 43 in., standing 27 ft. from a window in a 
darkened room. Outside the window was a mirror reflecting light from 
a bright sky into the room. Of all the light reflected from the mirror 
that only reached the telescope which passed through two pinholes in a 
piece of black paper supported in front of the mirror. The diameters of 
the pinholes were 1/30 and 1/20 in. respectively, and the distance 
between them 1/10 in. The iris diaphragm of an Abbe substage con- 
denser was supported centrally in a temporary mounting of wood fitting 
into the hood of the objective. When the diameter of the iris was 1/16 in., 
the two pinholes appeared, when seen through the telescope, as one dim 
hazy disc. When the diameter was 1/8 in., a smaller and more distinct 
disc w T as seen. When the diameter was 3/16 in., the disc was still 
