572 PHYSIOLOGY 



period by rays of a given intensity during a period A, and is then unillumin- 

 ated during a period B, if A 4- B is sufficiently small, e.g. in the case of the 

 disc if the rotation is sufficiently rapid, the sensation evoked is a continuous 



one and is equal to that which would be produced by a continuous stimulation 



^ 

 which is equal to - ' -. This fact is spoken of as Talbot's law. It enables 



us to produce a grey of any desired intensity by varying the relative sizes of 

 black and white sectors on a rotating disc. For complete fusion to take place 

 the period A + B need not be less than -04 sec. if we are using light of 

 moderate strength. With low intensity of illumination this value rises, i.e. 

 complete fusion is obtained with a lower rate of rotation of disc than when 

 we are using bright illumination. The value also varies according to the 

 colour of the light. Different colours require different times for their action 

 on the retina in order to produce the maximum sensation. If a spectrum be 

 exposed to the eye for a very short period of time it appears colourless and 

 shortened at the red end. If the period of exposure be increased the red and 

 blue ends are seen, but no other colour is perceptible. The sensations due to 

 the incidence of red rays attain their maximum in the shortest time, then 

 come blue rays, while the green take the longest time to attain their maxi- 

 mum. This difference between the time-relations and the sensations evoked 

 by the various rays accounts for the fact that on rotating a disc of alternate 

 white and black sectors at a certain rate the sectors appear blurred and bounded 

 by coloured fringes. 



FATIGUE 



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

 tion rapidly diminishes, i.e. the apparatus concerned in the production 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 phenomena 

 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, surf ace, 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 photo- 

 grapher, 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 



