7 
Objectives . ‘ - gothin. ~thin. ithin. fin. in. Lin. 
Thickness of focal plate i. : = i 7 ; : : 
or depth of prospective 
re 1 ie. 
150000 120000 60000 20000 6000 26500° 
It appears that perspective deepens with a given objective 
as the aperture is reduced. 
Dr. Goring first laid down the principle that ‘* penetration ” 
is always as the angle of aperture directly. This appears to 
the writer as the very opposite of the truth, and this prin- 
ciple is still entertained and applied as an obstacle to 
progress. By penetration he meant the resolving power of 
large aperture. 
The “resolution” of difficult structures can only depend 
upon sharp and distinct outlines, i.e. the exhibition of struc- 
ture as it really exists, without the false halo of aberration 
and the false shadows of complex illumination. If it were 
possible to construct an instrument that could imitate natural 
vision (as an object appears to the eye) under a small aper- 
ture, much vagueness of terms would disappear. It was 
found that only by large apertures could difficult test-objects 
be resolved. It was therefore thought essential to a fine defi- 
nition. Other causes, however, promoted the success, at that 
time unsuspected. 
Experiment 3.—A glass bead (;,'5th diameter) , formed by 
fusing a spun thread (,,,,th), is examined with an aperture 
of 120° objective. No black border appears till the aperture 
is reduced to 80°. It then increases in breadth as it is dimin- 
ished; the image of waving foliage glistening in the sun- 
beams becomes more distinct as the black band broadens ; 
and the light up to a certain point increases. 
Experiment 4.—Very fine hairs acquire a distinct sharp and 
black outline as the aperture is reduced ; penumbra vanishes, 
and the terminal central spectrum of intersection becomes 
gradually visible (9 a). 
Experiment 5.—The spines of the Podura scalewere resolved 
into three or four jet black beads with a reduced aperture 
(fig. 6) and a solar beam with careful corrections and oblique 
light incident in the direction of the longest diameter of the 
scale. During the reduction of the aperture in each of these 
cases the perspective evidently deepens. 
§ Il.—On the Use of a New Aberrameter for Testing Aber- 
ration and the Effects of Aperture. 
From the preceding and similar experiments it would 
appear that an instrument contrived to vary at will the 
