Note 16 — Chapter XVIII
Reading edition
Dr. Fripp's theory of the properties of the fish's eye is very plausible.
1st. That the fish's eye in its normal state is arranged for the vision of near objects, and that the great refractive power of a prolate spheroid lens, such as exists in the fish, is adequate to the production of a picture at short focal distances, even with rays of light passing through so dense a medium as water.
2nd. That there is no accommodation of the fish's eye for extended limits of vision.
3rd. That the passive state of the fish's eye, being that in which it is enabled to see objects near and at moderate distance, no active or physiological change for accommodation of sight for distant objects takes place or seems necessary.
The dioptric arrangement, being the reverse of that which obtains in animals where "accommodation" is observed, and in whom the passive state is that of vision, arranged for distant objects, while the active state is that of vision accommodated at will for near objects.
4th. That the vascular distribution of the choroid vessels has no relation to any movement of the lens or change of its shape, but is arranged to meet the changes of static condition of the circulating fluid, and of dynamic force exerted by the heart under varying pressure from without; and that by such an arrangement, protection to the delicate tissues of the eye is afforded by a compensating balance between the tension of the blood within the vessels and the external pressure exerted upon them.