Stonsy—Linutation of Insect Vision. 231 
diameter of the object-lens. This furnishes a boundary within 
which the central spurious disk lies, and up to which its faintest 
outlying portion barely extends. It also fixes the minimum visibile 
with that aperture, since two points would have begun to be 
blurred into one another if so close that the middle of the spurious 
disk of each lay on the first dark ring of the other. Let us then 
put into this formula, #=1’=-00029 in circular measure (this is 
the limit already fixed by the rods and cones), and \=°6 of a 
micron (which is the wave-length of yellow light). We thus 
find 
6 
00029 = (1:22) me 
whence A =2524 microns, which is very nearly 1/10th of an inch. 
This, then, is the diameter of the pupil of the eye when of such 
size as to put the same limit on the visibility of small objects as 
the rods and cones do. Now, this is about the size to which the 
pupil of the eye shrinks when we scrutinize well-illuminated 
objects, and is the smallest to which it can be allowed to shrink 
without interfering with the vision of minute detail, by placing a 
further restriction beyond that imposed by the layer of rods and 
cones." 
Again, the eye viewed as an optical instrument is far from 
perfect. Its chromatic defect may be detected by placing the 
finger horizontally in front of the eye, and looking just over it at 
the bar of a window. In this way the window-bar is viewed 
through the upper half of the pupil, and is then seen to be 
bordered with colour. Finally, the spherical aberration? of the 
eyes becomes conspicuous when we view a considerable star or 
planet with one eye. Instead of being seen as a point, it is seen 
1Tt might be thought that with the more dilated pupil which we have in faint light, 
we could see more detail. But the reverse is the case; for instance, the two small 
double stars «1 and e2 Lyre are more than 3’ asunder, and yet, in consequence of their 
faintness are nearly at the limit of what a very good eye can see distinctly as two 
objects. To eyes that are fairly good they appear as one object elongated, while per- 
sons with only tolerably good sight do not even see the elongation. 
* Tf a sphere be drawn round a point of the image formed by light of one wave- 
length, to represent the crest of one of the luminous waves advancing towards that 
point, the whole of the crest should reach that sphere at the same instant of time. 
There are, however, usually little deviations of some parts of the crest of the wave 
from this sphere, which defect is called spherical aberration. 
