88 COLOUR VISION 



image. The secondary image fades gradually and slowly. In the case 

 of the travelling object the secondary image appears abruptly at a brief 

 interval after the last bands of the primary image, and fills a part or 

 the whole of the track of the image. If the object light is coloured and 

 of high intensity the secondary image is in both cases tinged with the 

 same colour ; but if the object light is coloured and of low intensity the 

 secondary image is grey in the case of blue and green light, but is absent 

 in the case of pure red light. The secondary image is the ordinary 

 positive after-image (vide infra). 



The interval between the primary and the secondary images is not 

 always completely dark. As the rate of movement of a travelling 

 object light is increased dim grey bands appear in the interval. They 

 are dimmer and broader than the leading bands and differ markedly 

 in quality, the leading bands being yellowish-white, these others a 

 neutral grey. The interval-bands show the following peculiarities : 

 (1) they are brightest when the object light is green, absent when it is 

 red ; (2) they are absent where they cross the fovea, even with a green 

 object light; (3) they have the neutral ghostly quality, inclining 

 towards blue, characteristic of the scotopic spectrum ; (4) they are 

 absent when the eye is light-adapted. 



It is probable therefore that the primary response is a response 

 of the photopic mechanism, any response of the scotopic mechanism 

 being obscured by the preponderant photopic effect. On the cessation 

 of the photopic response the scotopic response becomes manifest under 

 favourable circumstances. There is reason to believe that the latent 

 period of the photopic reaction is shorter than that of the scotopic by 

 about yV sec. 



The secondary image is similarly complex. With an object light 

 of low intensity it is a pure scotopic reaction, as shown by its grey 

 quality and its absence with a pure red object light. With a brighter 

 object light it is a combined photopic and scotopic reaction, the photopic 

 preponderating with strong lights, as shown by the colour and saturation 

 of the secondary image. 



McDougall has further shown that the bright initial reactions which 

 constitute the primary image exert an inhibitory effect upon the im- 

 mediately succeeding reactions. BidweWs ghost is the last of the series 

 of pulses of sensation, the intermediate members being thus inhibited. 

 In its typical form it is a pure scotopic reaction, but that it is not 

 necessarily so is shown by the fact that it can be obtained by a pure 

 red object light of high intensity and does not then jump the fovea. 



