132 ESSENTIALS OF PHYSIOLOGY. 



If the position of the light is altered, that of the images will also change. 

 The degree of displacement of the light and that of the displacement 

 of the images, as well as the distance of the latter from the eye, 

 being known, the distance between the retinal vessels and the layer 

 of the retina which is stimulated by the shadows can be calculated ; 

 and it is found in this way that the structures which are stimulated 

 correspond in position with the layer of rods and cones. The 

 appearance of Purkinje's images is due to the fact that the light, 

 falling from an unusual direction, casts the shadow of the vessels on 

 a part of the retina unaccustomed to such a stimulus. 



Inasmuch as the rods and cones are of different structure, it is to 

 be expected that their functions also differ, and there is evidence that 



FIG. 42. (Starling's Principles of Physiology.} 



this is the case. The cones are most abundant in the central area of 

 the retina, and the rods are in greater proportion in the peripheral 

 part. Associated with this distribution is the fact that central vision 

 is more distinct in ordinary light and peripheral vision more distinct 

 in dim lights. Thus a star which is seen by indirect, or peripheral, 

 vision, may be invisible when the eye is directed towards it. This 

 fact suggests that the cones are adapted for vision in good light and 

 the rods for use in dim light, an assumption which is supported by a 

 comparative study of the retina in animals. In most birds, which 

 go to roost when twilight falls, or even when the sky is obscured by 

 heavy clouds, cones only are present. In owls and bats, on the other 

 hand, which are nocturnal in their habits, there are only rods. 

 The study of the field of vision for colour shows that whereas the 

 central area of the retina is_ jgsponsi ve to all coloured rays, the 

 periphery is colour-blind. Moreover^all parts of the retina are colour- 

 blind in dim light. 



It is justifiable, therefore, to believe that the cones are functional, 

 in good Jigfet, an d are responsive to stimulation by white and coloured 

 rays, whereas the rods come into play in dim lights and are colour- 



