578 THE SENSES AND SENSORY ORGANS. 
Exner ascribes the appearance to a reflection from a tapetum 
behind and between the great rods; he further considers that 
the disappearance of the reflection is due to an alteration in 
the disposition of the iris pigment. 
There are, I think, many objections to the view that the 
reflection originates from the great rods or from the tracheal 
tapetum, if this term is to be applied to the tracheal network 
between and behind the great rods, especially as this tapetum 
is least developed in those very insects in which the reflection 
from the eye is most brilliant, and is most developed in those 
insects, as the majority of Diptera, in which there is no 
vestige of luminous reflection. The insects in which the 
luminous reflection is most brilliant are, however, all dis- 
tinguished by the large size of their crystalline cones. Now, it 
is manifest that the reflection of the light which enters the eye 
will be most marked where the refraction of the rays is most 
powerful, and where the highly-refractive medium is concave 
towards the source of light. The convex surface of the cornea 
disperses the light which is reflected from it; but the apex of 
the crystalline cones is virtually a concave mirror, and the light 
reflected from it through the corneal lens will leave the eye 
as a pencil of parallel, convergent, or slightly divergent 
rays. 
Such a reflection will only occur from the internal surface 
of the crystalline cones where these are not covered externally 
by dark or black pigment. In the nocturnal Lepidoptera 
which have been kept in the dark the iris pigment is with- 
drawn from the apex of the cone, so that a considerable por- 
tion of its surface acts as a concave reflector. I find that 
the reflection from this surface corresponds with a luminous 
image of the cone apex, subtending an angle of 20° to 25° 
from the optic centre of the corneal lens. As the adjacent 
lenses have an angular divergence of about one degree, the 
images produced will overlap and intensify each other. 
Hence the light which emerges from the insect’s eye gives an 
image of a bright disc, apparently situated behind the cornea. 
The diameter of this disc is equal to that of from twenty to 
