THE ORGANS OF SENSE. 135 



after-irnage, and is best seen on waking from sleep. The same per- 

 sistence of sensation is the cause of the solid appearance of the spokes 

 in a rapidly revolving wheel. The normal duration of the visual 

 sensation can be measured by means of revolving discs on which are 

 black and white sectors. If the gaze be directed to such a disc while 

 it is revolving slowly, the separate sectors can be distinguished. With 

 an increased speed of revolution the disc appears to be of a uniform 

 shade of grey, at first producing a sensation of "nicker," and later, 

 as the rate of rotation increases, and fusion is complete, showing a 

 uniform and steady grey appearance. If the rate of rotation at which 

 complete fusion occurs and the size of the sectors are known, it can 

 be calculated that the duration of each impression on the retina after 

 the withdrawal of the stimulus is about one-fiftieth of a second. The 

 fusion of sensations is comparable with the fusion of single muscular 

 contractions to produce tetanus. 



COLOUR VISION. 



The various colour sensations are due to stimulation of the retina 

 by rays of different wave-length, and it has long been a subject of 

 discussion as to whether the various qualities of visual sensation are 

 associated with stimulation of different end-organs or different 

 chemical substances in the retina. Many theories have been put 

 forward, but none of these accounts for all the facts, and only two 

 need be mentioned here. 



The Young-Helmholtz theory postulates the presence in the retina 

 of three photo-chemical substances, one of which is susceptible to 

 stimulation by the spectral red rays, and to a diminishing extent by 

 the other rays from the orange to the violet ; a second, which is affected 

 chiefly by the grejen rays and to a less extent by those toward either 

 end of the spectrum ; and a third, which is mostly affected by the 

 violet rays, and to a diminishing extent by the remainder of the 

 spectrum from indigo to red. Stimulation of all three substances to 

 an equal extent excites the sensation of white. Stimulation mainly 

 confined to the red substance gives the sensation of red, whereas equal 

 stimulation of the red and green substances with slight affection of the 

 violet substance gives rise to the sensation of yellow. The other 

 colour sensations are excited in the same way by varying degrees of 

 stimulation of the three substances. 



On this theory there are three primary sensations, and the 

 hypothesis finds its chief support in the facts connected with colour 

 blindness. Total colour blindness is rare, but about 4 per cent, of 

 European males are partially colour blind, the commonest form being 



