FURTHER REMARKS ON THE COMPOUND EVE. 577 
c. Accessory Optic Phenomena in the Compound Eye. 
Exner says: ‘It is manifest that, in so complex a structure 
as the compound eye, phenomena must occur from refraction, 
reflection, absorption, etc., which are not directly connected 
with vision. Owing to the great variations in the structure of 
this organ, these phenomena vary in different species, but cer- 
tain phenomena are typical, especially the internal light and 
pseudo-pupils,’ which he proceeds to discuss. 
The Internal Light is a brilliant metallic reflection from the 
interior of the compound eye, which occupies a greater or less 
surface of the cornea opposite to the eye of the observer. 
It is characteristic in the Sphingide and Noctuz, and is 
apparently confined to nocturnal and semi-nocturnal insects. 
If a large moth (a Sphinx or Noctuid) is kept in the dark 
for a few hours, the eyes are seen to shine like those of a cat. 
In a bright light the luminous reflection gradually contracts in 
diameter, becomes more and more feeble, and disappears in a 
shorter or longer time—from a minute to five minutes in day- 
light—and more rapidly the brighter the light. The pheno- 
menon only recurs after the insect has been kept in the dark 
for from half an hour to three or four hours. 
Leydig described this reflection as similar to a glowing coal, 
and ascribed it to a reflection from a tapetum. He says 
that the tapetum is tracheal, and considers that the reflection 
either depends on a fuller condition of the tracheal vessels, or 
on a dilated condition of the pupils or perforations in the sub- 
corneal pigment. 
Max Schultze regards the reflection as being produced by the 
laminated structure of the visual rods, which he describes, and 
not by the tracheal vessels. 
Exner [252] says that, by the help of an ophthalmoscope, 
most faceted eyes, although not all, exhibit a reflex—that is, 
the light which enters the eye returns in exact analogy with 
what happens in Vertebrates. This reflex is limited to a 
circular spot, which is nearly or absolutely identical with the 
optical appearance known as a pseudo-pupil. 
38—2 
