112 
ANNIVERSARY ADDRESS 
degrees of light, and that they can produce nothing worthy the 
name of an image. 
In the isapod crustaceans are found groups of single eyes, which 
might he considered as intermediate between a single eye and a 
compound eye. L an dois,* * * §1 however, in 1866, drew attention to eyes 
of caterpillars as a link between the two sets of visual organs. The 
subject appears to have been neglected until, in 1884, Lowne, in 
the ‘ Transactions of the Linnean Society,’ f drew attention to these 
so-called “compound single eyes” in noctuid caterpillars. He 
describes beneath the lens of each eye a fusiform spindle, and 
attaches to it a function which we will explain hereafter. Several 
of these eyes are united by branches from the optic nerve. 
The Compound Eye. 
The outer surface of the cornea of the compound eye is divided 
into very many minute facets, behind each of which is a lens. 
Close behind this lens is a structure called the crystalline cone, J the 
apex of which is surrounded with pigment. This cone closes the 
top of a long narrow chamber, at the bottom of which is a spindle. 
Each corneal lens, crystalline cone, and spindle is enclosed in a 
common sheath, and the whole rests side by side with the other 
similar portions of the eye, on a membrane, at the back of which is 
the optic nerve. The long columns or rods are generally considered 
to be the endings of the optic nerve. The inner extremity of the 
long rods is usually surrounded with pigment, and they are pene¬ 
trated by many small tracheal vessels. How, if we look at the 
eyes of a moth which has been kept some time in darkness, we see 
a brilliant luminosity, which after a little time fades away. The 
luminosity is explained by the reflection from these tracheal vessels, 
and its gradual disappearance is due to the contraction of the pig¬ 
ment surrounding the tops of the crystalline cones. Pigment, as 
already stated, is exceedingly sensitive to light, and just as the 
granules are brought closer together in the skin of frogs and fishes, § 
by the action of the sun, so are the pigment-granules on the top of 
the crystalline cone brought closer together and so shut off the 
rays of light. 
The manner in which the compound eye subserves the function 
* ‘ Zeitschrift f. Wissenschaftliche Zoologie,’ Bd. xvi, p. 27 ; and ‘Ann. Mag. 
Nat. Hist.,’ 3rd Ser., vol. xix, p. 61 (1867). 
f 2nd Ser., Zoology, vol. ii, p. 395. 
+ In some insects the crystalline cone is replaced by a capsule containing fluid. 
§ The direct effect of light on pigment is perceptible, but it must not be 
confounded with the protective results of the chromatic function to which I refer 
in my conclusion. 
