VISUAL SENSATIONS 573 



time, however, we begin to distinguish objects more clearly. Any slight 

 defects in the dark chamber begin to make themselves apparent, such as 

 entry of light under the door and through cracks or nail- holes in the ceiling 

 or walls. By direct measurement it can be shown that within ten minutes 

 after passing from daylight into complete darkness the sensitiveness of the 

 retina increases twenty-five- fold, and after two hours' exposure to complete 

 darkness thirty-five-fold. On the other hand, on coming out of the dark 

 room we are at first dazzled by the flood of light with which we appear to be 

 surrounded ; the pupils constrict to their utmost; and accurate vision is 

 impossible on account of the excess of light which seems to pour into our 

 eyes. Very shortly this condition passes off, and within five minutes 

 vision is once more normal, and the ordinary size of the pupils re- 

 established. 



The process of adaptation affects not only the quantitative relation 

 between the intensity of the stimulus and the resulting sensation, but deter- 

 mines also a qualitative alteration in the reaction of the retina to light. This 

 is especially marked in the case of colours. On going into a flower-garden 

 on a summer morning, when dawn is just beginning, although all objects 

 in the garden can be clearly distinguished, there is a striking difference 

 in its colour- tone as compared with that which it presents in daylight. The 

 scarlet geraniums have disappeared. On close examination this disappear- 

 ance is found to be due to the fact that the flowers are dark, i.e. the light from 

 them does not stimulate the retina at all. The other coloured flowers can be 

 distinguished, but only in shades of grey. With a very little increase in 

 illumination the blue flowers come into evidence and the prevailing tone of 

 the garden is cold, made up as it is of greens, blues, and greys. With in- 

 creasing illumination the reds finally make their appearance. After long 

 exposure to the darkness of night the eyes have become dark-adapted. The 

 same behaviour of the dark- adapted eye may be demonstrated in the 

 laboratory. If a person who has been in a dark room for half an hour ob- 

 serves a spectrum of low intensity the whole spectrum appears colourless, its 

 red end being cut off. The distribution o luminosity over the spectrum is 

 also altered. Whereas the spectrum to the normal light-adapted eye 

 appears brightest in the yellow between the lines D and E, the spectrum of 

 low intensity to the dark-adapted eye has its point of greatest luminosity in 

 the green. 



This striking difference between the light-adapted and the dark-adapted 

 eye does not apply to small objects the image of which, when the vision 

 is directed towards them, will fall entirely on the fovea centralis of the retina. 

 In the dark- adapted eye, the sensitiveness of the central spot of the retina 

 is not nearly so great as that of the more peripheral portion. On a dark 

 night we are often able to distinguish a star which, however, disappears as 

 soon as we turn our eyes so as to bring its image on the central spot of the 

 two retinae. Moreover the qualitative change in relation to the colours 

 observed in the dark-adapted eye does not apply to the fovea centralis. 

 In a dark room a small spot of light, whatever its colour, when the visual 



