VISUAL SENSATIONS 639 



FATIGUE 



If a constant stimulus be prolonged, the intensity of the resulting 

 sensation rapidly diminishes, i.e. the apparatus concerned in the pro- 

 duction of the sensation shows signs of fatigue. This diminution in 

 the intensity of sensation may be observed so early as one-fifth of a 

 second after the beginning of the stimulus. Connected with this 

 fatigue of the retina is the phenomenon known as the ' negative after- 

 image.' If we look at a bright spot or source of light for some seconds 

 and then turn our gaze to a uniformly illuminated white surface, we 

 see in the middle of the white surface, i.e. at the point of fixation, a 

 dark image of the bright object which we had previously looked at. 

 The stimulus applied to all parts of the retina in this case is uniform. 

 Certain elements, however, i.e. those which had been previously 

 stimulated by the bright object, are fatigued. Their response is 

 therefore less than that of the surrounding untired retinal elements, 

 and the resulting sensation is a dark image, which is referred to that 

 part of the white surface from which proceeds the light falling on the 

 fatigued elements of the retina. 



ADAPTATION 



It is common experience that our eyes have the power of adapting 

 their sensitiveness according to the degree of illumination. When 

 we pass from daylight into a dark room, such as the developing chamber 

 of the photographer, it is at first impossible to distinguish any objects 

 even by the dim red light coming from the photographer's lamp or the 

 window. After a short 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 sur- 

 rounded ; 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 rela- 

 tion between the intensity of the stimulus and the resulting sensation, 

 but determines also a qualitative alteration in the reaction of the retina 

 to light. This is especially marked in the case of colours, On 



