THE RETINA. 169 



shorter wave lengths; a green which appears to the colour-blind eye to match a 

 certain yellow seems to the normal in daylight to be not only of a different colour 

 from the yellow chosen, but also less bright than it. The sight of people who have 

 this defect is easily fatigued in bright light and is much less acute than the normal. 

 In the dark it far surpasses the normal in accuracy. Many colour-blind people 

 have a blind spot in the fovea, so that they see nothing in the direct line of vision 

 but only in the peripheral field. It is thought that the condition is one in which 

 the rods mainly function. The parallel between these eyes and the normal dark- 

 adapted eyes is not complete. The vision of the colour-blind is much the 

 more acute and is often enough so to enable the subjects to read, which cannot 

 be done with normal dark-adapted eyes. 



