946 



A MANUAL OF PHYSIOLOGY 



retina, they are supposed to be more rapidly built up in the sur- 

 rounding parts, and in this way many of the phenomena of 

 simultaneous contrast receive an easy and natural explanation. 

 The same is true of the simpler phenomena of after images or 

 successive contrast. But in applying the theory to the more com- 

 plicated phenomena difficulties soon emerge, which, to say the 

 least, are not less formidable than those connected with the 

 Young-Helmholtz theory. Neither theory, in short, can be 

 considered more than a partially successful attempt to grapple 

 with a very complex mass of facts. Each, however, has been 

 fruitful in leading to the discovery of new facts a great merit 

 in a scientific hypothesis. 



Sensibility of Different Parts of the Retina^Perimetry. 



The perception of 

 colours, like the per- 

 ception of white light, 

 is not^equally distinct 

 over the whole retina. 

 We have repeatedly 

 had occasion to refer 

 to the fovea centralis 

 as the region of most 

 distinct vision^; but it 

 would be a mistake to 

 suppose that it is 

 therefore necessarily 

 more sensitive than 

 the rest of the retina. 

 As a matter of fact, 

 when the minimum 

 intensity of white light 

 which will cause an 

 impression at all is 

 determined for each 

 portion of the retina, 

 it is found that the 

 fovea centralis requires a somewhat stronger stimulus than the 

 zone immediately surrounding it. Objects only a little brighter 

 than the general ground on which they lie e.g., very faint stars 

 are best seen when the eye is directed a little to one side. This 

 has been attributed to the absence of visual purple from the 

 fovea, in accordance with the theory previously alluded to that 

 the visual purple acts as a mechanism which ' adapts ' the retina 

 for the perception of light of varying intensity. But, with 

 this exception, the sensibility of the retina diminishes steadily 

 from centre to periphery, both for white and for coloured light. 



FIG. 415. PRIESTLEY SMITH'S PERIMETER. 



K, rest for chin ; 0, position of eye ; Ob, object, 

 white or coloured, which slides on the graduated 

 arc B ; /, point fixed by the eye. 



