ioi8 



THE SENSES 



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 perception 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 some- 

 what stronger stimulus 

 than the zone immedi- 

 ately surrounding it. Ob- 

 jects 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 attri- 

 buted to the absence of 

 visual purple from the 

 fovea, in accordance with 

 the theory previously 

 alluded to that the visual 

 purple acts as a mechan- 

 ism which ' adapts ' the 

 retina for the perception 

 of light of varying inten- 

 sity. But, with this ex- 

 ception, the sensibility of 



the retina diminishes steadily from centre to periphery, both for 

 white and for coloured light. 



When the eye is fixed, and the visual field that is, the whole 

 space from which light can reach the retina in the given position, or, 

 what comes to the same thing, the projection of the visual field on 

 the retina by straight lines passing through the nodal point- 

 explored by means of a perimeter (Figs. 430, 431), it is found that, 

 under ordinary conditions, a white object is seen over a wider field 

 than any coloured object, a blue object over a wider field than a 

 red, and a red over a wider field than a green object. The exact 

 shape, as well as size, of the visual field also differs somewhat for 



F lg . 430. -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. 



