THE SIMPLE AND COMPOUND ETES OF INSECTS. 
581 
The type of the compound eye in the Wasp is the same as that of the eye of 
Tipula,, but the two differ in the following points : — 
The curvature of the general cornea is so slight that the visual axes of adjacent 
facets in the centre of the cornea only make angles of 8' with each other. The 
facets are only 2 q 0 0 -th of an inch in diameter, but they are yiroo'th of an inch in 
thickness, and consist of numerous layers. The approximate radius of curvature of 
the outer surface of a facet is ts'o o th, and that of the inner surface is ^- 5 Voth of an 
inch. 
The refractive index of the material of which the compound cornea is formed does 
not differ materially from that of Canada balsam : this is easily seen in specimens 
mounted in fluid balsam. In order to determine the index of refraction with the 
greatest accuracy, I found the focal lengths of the lenticular facets of the cornea of 
a Hornet first in air and then in water, thus eliminating the radii of curvature. By 
this means I calculated the refractive index to be 1'53. The great difficulty is the 
determination of the real focus with sufficient accuracy, but the results are sufficiently 
accurate to give an approximate idea of the position of the focus in the eye. These 
results give 2 "ooth of an inch as the distance of the focus behind the inner surface of 
the cornea, so that the rays may be considered as approximately parallel to the axes 
of the rod-cells. 
As in Tipula, there are sixteen rod -cells ( a ") behind each facet. There is also a 
small highly refractive globule of a dark purple colour, and a facellus (f) very similar 
to that in the eye of Tipula behind the rod-cells. All these structures are sur- 
rounded by so much deep violet pigment in my preparations that the details can 
only be observed with considerable difficulty. 
IIJa. On the Compound Eye of Formica rufa. (Figs. 6, 7a, and 8a.) 
My description and figures of the eye of this insect are taken from the eye of the 
mature female imago. 
The eye of this Ant differs but little from that of the Wasp. The corneal facets 
are larger, measuring yygo'th of an inch in diameter, but are not more than - 2 ooo-th of 
an inch in thickness. The spherules beneath the cornea are colourless. The rod- cells 
(a ) are imbedded in a large quantity of deep violet pigment (fig. 7a ) ; they are 
TTJooTjth of an inch in diameter. The facellus (f) is shorter and wider, and consists 
ox more rod-like cells than I have observed in the facellus of any other insect : there 
are at least twelve cells ; it is surrounded by a layer of deep purple pigment. The 
chamber in which the rod-cells lie is lined by deeply pigmented rod-like cells which 
differ from those in the centre of the chamber in the extent of their pigmentation and 
in not being connected with the facellus, so that in some of my sections in which the 
facellus and the deeper parts have been torn aw T ay the pigmented rods of the periphery 
of the chamber alone remain. Under these circumstances the eye appears to be 
