460 
POPULAE SCIENCE EEVIEW. 
This is by reason of the structure of the eye as an optical 
instrument, and because the rays of light in entering and in 
emergingfrom it undergo refraction, according to definite laws. 
The light which penetrates the eye traverses the transparent 
retina, producing the impression necessary for sight, and is 
partly absorbed by the black pigment of the choroid ; but a 
great number of the rays are reflected ; for here there is no 
exception to the general rule tliat some of the rays of fight 
falling upon any substance are always reflected. These rays, 
in returning, are refracted through the vitreous body and lens, 
just as they were in entering the eye, with the object then of 
causing them so to converge as to produce upon the retina a 
clear and definite image of whatever external object they 
started from. Similarly, then, on their emergence they are 
refracted chiefly by the lens and cornea, so as to form an 
image in the outer air, the emergent rays coinciding in their 
path with that which they took when entering, and the image 
formed in the air being conjugated with the retinal image ; 
being formed therefore on the same side, varying with the 
position of the lens and object, and the accommodation of the 
eye. Thus, then, to perceive this aerial image, derived from 
the retinal reflection, the eye of the observer needs to be 
placed in the axis of the converging rays ; but since this is 
also the axis of the entering rays, he will of necessity in that 
position cut off those rays altogether of the fight proceeding, 
say, from a lamp, or the source of fight opposite to the eye to 
be illuminated. 
The problem to be solved consists, then, in the simple 
illumination of the eye to be observed by a source of light so 
arranged that the eye of the observer can be placed in the 
axis of the rays entering and emerging without intercepting 
those rays. This may be most conveniently effected by 
placing the source of fight aside of the eye to be observed, and 
observing through a pierced concave mirror, which reflects 
that fight into the eye. We can then, by looking through the 
central aperture of this mirror, place ourself in the path of 
the entering and emerging rays. The mirror becomes the 
