vii RETINAL EXCITATION 361 



distinguish them as faint shining dots on a black ground. But if 

 we look at one directly we at once cease to see it, while the other 

 discs, the images of which fall on the peripheral parts of the 

 retina, remain visible. The more perfectly the eye is adapted to 

 darkness the brighter will the objects appear with eccentric, and 

 the less visible with central vision. 



While, therefore, in day (photopic) vision, while the eye is 

 adapted to light, the fovea centralis is the most sensitive part of 

 the retina ; in twilight (scotopic) vision, with the dark-adapted 

 eye, the fovea is in a state of physiological hemeralopia, as compared 

 with the peripheral parts of the retina. The progressive increase 

 of retinal sensibility in the dark thus predominantly affects the 

 peripheral zones, and only involves the central region of the retina 

 to a much less extent. 



Later on we shall return to the physiological value of this fact, 

 which seems to be correlated with the formation of visual purple 

 as found exclusively in the rods. But it may be added that 

 these positive and negative variations of the sensibility of the eye 

 to light afford an easy explanation of the everyday fact that we 

 are able to tolerate considerable objective variations in the intensity 

 of illumination without difficulty or inconvenience. 



VII. It is an important fact that a stronger stimulus is 

 required for chromatic sensations i.e. to see and distinguish 

 colours than for achromatic or colourless sensations. Under 

 weak illumination we can neither see nor distinguish the colours 

 of objects : everything looks grey and obscure. The eye behaves 

 as though it were totally colour-blind ; and this is the character- 

 istic of scotopic (twilight) vision. 



Too vivid illumination also diminishes the power of discrimin- 

 ating colours ; chromatic sensibility becomes increasingly fainter, 

 and is finally lost ; yellow readily passes into white. 



It may therefore be said that clear, distinct chromatic sensa- 

 tions only occur with light of moderate intensity ; with stronger 

 or weaker light they become achromatic. This proves the uniform 

 character of the light stimulus, considered objectively. 



The relative brightness of colours, again, changes considerably 

 with the degree of illumination. Yellow, orange, and red are 

 the brightest colours in daylight, while with the same degree of 

 illumination green, blue, and indigo appear comparatively dark". 

 In twilight, on the contrary, yellow, orange, and above all red, 

 become dimmed, while green and blue are relatively bright. This - 

 fact is known as Purkinje's phenomenon, as he first discovered it. 

 It can easily be confirmed on looking at the spectrum of daylight 

 or gaslight, under strong or weak illumination, with the eyes 

 adapted to light or darkness. 



On looking with light-adapted eyes at a well-illuminated solar 

 spectrum, it is at once evident that the different colour-bands are 



