560 THE SENSES AND SENSORY ORGANS. 
to 1°55. Ina similar manner I determined the refractive index 
in the cornea of the Hornet, and found it to be 1°53 [219]. 
It is well known that a lens composed of layers increasing 
in density towards the centre has a higher refractive index 
than the highest refractive index of any of its layers, and 
Exner accounts for the high refractive index of the entire 
crystalline cone in Dytiscus on the same principle. He 
believed it to be a cone consisting of a series of envelopes 
increasing in refractive power towards its axis. There is little 
doubt, I think, that this affords a true solution of the dis- 
crepancies previously observed. 
Exner terms such a cylinder a refractive cylinder, and shows 
that such cylinders act exactly as convex lenses do, and that 
the focus of a refractive cylinder is a recurring function of 7 
where c is a constant, and / the length of the axis of the 
cylinder within certain limits (see p. 572). Cylinders have 
been made of glass annealed in a special manner which 
exhibit all the properties of Exner’s refractive cylinder. 
Exner believes that the subcorneal image lies in the middle 
of such a cylinder, the crystalline cone, and that rays which 
were parallel before entering the refractive media of the insect’s 
eye leave the inner extremity of the crystalline cone as parallel 
rays. If this were the case, it is difficult to understand the 
optical purpose of the refractive media, as no other result would 
be attained than a diminution of the amount of light falling 
upon the retinal end organ, unless such parallel rays were sub- 
sequently brought to a focal point, since the density of a pencil 
of light can only be increased by the convergence of the rays of 
which it is composed (see page 575). 
The Dioptric Theory, which I enunciated in 1884 [283], is 
that the pencils leaving the crystalline cone are brought to a 
second focus by the rhabdome, which acts as a lens; these 
pencils may be either divergent, or parallel pencils as Exner 
supposes, and in this way a magnified image of a part of the 
subcorneal image is formed at the inner extremity of the 
rhabdome, and as there is a double reversal, it is directed in 
the same manner as the external object and is an erect image 
