Chap. 17 



RESPONSIVENESS THE SENSE ORGANS 



325 



View Camera 



sKuttec JidpKraigrn lens supporting photosensitive bldcKcned 



atructunz film surface 



lens scleral 

 coat 



retina. 



cKoroidL 



6TS>* 



Fig. 17.14. Diagrams illustrating the similarities of the human eye and the cam- 

 era. In both eye and camera there are two main processes: a physical one by which 

 rays of light are directed to a focus through one or more lenses and a chemical 

 reaction between light and light sensitive substances. In the eye, the latter occurs 

 in the rod and cone cells of the retina; in the camera it occurs within the coating 

 of the photosensitive film. In the camera, the glass lens can be moved nearer or 

 farther from the film in focusing. In the human eye, the shape of the lens as well 

 as its position are changed. In the camera, the amount of light is changed by the 

 diaphragm; in the eye, by the iris. (Courtesy, Ham: Histology, ed. 2. Philadelphia, 

 J. B. Lippincott & Co., 1953.) 



"It seems likely now that photoreception, visual [image forming] or photo- 

 tropic [light turned] throughout all living organisms may be founded chem- 

 ically upon this single group of substances." 



The Human Eye 



The Eye and the Camera. The eye is a complex organ that takes pictures 

 again and again on the same light receptors (rod and cone cells) that re- 

 generate their own sensitivity and after one exposure are instantly ready for 

 another. The camera is a complex contrivance that takes pictures when prop- 

 erly operated and its light receptor, the coating of the film, can only be 

 exposed once because it never regenerates (Fig. 17.14). The evolution of 

 eyes has been going on many millions of years. The history of the camera 

 has been relatively short, even including the early suggestions of it in the 



