Note 10 — Chapter III · THE PHYSIOLOGY OF THE CRAY­FISH—THE MECH­A­NISM BY WHICH THE LIV­ING OR­GAN­ISM AD­JUSTS IT­SELF TO SUR­ROUND­ING CON­DI­TIONS AND RE­PRO­DUCES IT­SELF.

Since the visual rods are strongly refracting solids, and not empty tubes, the diagram given in fig. 29 does not represent the true course of the rays, indicated by dotted lines, which fall obliquely on any cornea of a crayfish’s eye. Such rays will be more or less bent towards the axis of the visual rod of that cornea; but whether they reach its apex and so affect the nerve or not will depend on the curvature of the cornea; its refractive index and that of the crystalline cone; and the relation between the length and the thickness of the latter.