SOME INTERACTIONS WITH LIVING MATTER 



85 



point 

 light 

 source 



(the object) 



ornea 

 aqueous humor 



ciliary muscles 



optic nerve 

 Figure 4-7. Architecture of the Left Eye, Viewed from Above. 



liquid crystal materials and are, of course, transparent. About 48 per cent 

 of 7 reaches the retina. The iris acts as would the diaphragm of a camera, 

 controlling the area of the pupil, and hence the total energy admitted. 



The incoming light, which is usually divergent from the source, is focused 

 on the retina by the lens. The distance, q, between the lens and the image (of 

 the light-source) on the retina is constant, but the lens-to-object distance, p, 

 may vary widely from about 4 in. to a mile. To be versatile, then the focal 

 length, /, defined as 



_L _L J_ 



/ ~ P + q 



must be adjustable if objects at different distances are to have sharp images 

 on the retina. Now the focal length depends upon the geometry of the lens: 

 a thick lens will have a short focal length, and a thin lens a long focal length. 

 Because the lens is a liquid crystal much like jelly, its shape can be changed 

 by the tension exerted by the ciliary muscles. This tension is in turn con- 

 trolled by a nervous signal fed back from the retina, the cells of which esti- 

 mate the sharpness of the image. This process is known as accommodation. 



Photosensitive Cells 



The focused light falls on two types of cells on the retina, rod cells and 

 cone cells, named because of their shape. The rod cells (scotopic vision) are 

 the more sensitive to light, and distinguish for us light from dark when the 

 intensity is very low (twilight vision). On the other hand the cone cells 

 (photopic vision) are less sensitive, can resolve large amounts of light into its 

 components, and therefore detect details of the image, such as shape and 

 color. 



The photosensitive cells are present in large numbers, estimated at 

 126,000 cells/mm 2 . Most of the cone cells are clustered close together about 



