638 PHYSIOLOGY 



and not instantaneous. With further increase in rate the whole disc 

 takes a grey colour, which, however, oscillates or ' flickers/ and with 

 a still further increase the * flicker ' disappears and the disc has a 

 uniform grey appearance. The phenomenon is analogous to the 

 phenomenon observed in muscle as the result of intermittent excita- 

 tion. Each single shock gives rise to a contraction which is more 

 prolonged than the shock itself. When the shocks are repeated 

 we obtain, according to their frequency, a series of single contrac- 

 tions, a partial fusion of contractions, so that an imperfect tetanus is 

 produced (' flicker '), or finally with a certain rate of interruption of 

 the stimulus complete fusion of contraction, i.e. complete tetanus. 

 We see therefore that when a portion of the retina is excited 



for a certain period by rays of a 

 given intensity during a period A, 

 and is then unilluminated during 

 a period B, if A -|- 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 stimu- 



j^ 

 lation which is equal to , 



A. ~\ _t> 



This fact is spoken of as Talbot's 

 ^ aw< ^ 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 maximum. 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. 



