268 



Mr. Shelford Bidwell. On the Negative 



" On the Negative After-images following Brief Retinal 

 Excitation." By Shelford Bidwell, M.A., LL.B., F.B.S. 

 Received April 15 —Read May 13, 1897. 



The experiments to be described were undertaken with the object 

 of ascertaining the true nature of certain phenomena referred to in a 

 recent communication, which had been left over for further examina- 

 tion. For full details reference should be made to the former paper,* 

 but it will be convenient to recapitulate very shortly one or two 

 of the results which were arrived at. 



It was found that if the image of a bright object upon a dark 

 background were suddenly formed upon the retina, the object gener- 

 ally appeared to be surrounded for about one-tenth of a second by a 

 narrow red border. As a consequence of this circumstance, a black 

 line drawn upon white paper appeared for a moment after it was first 

 exposed to view to be entirely red. Methods were described by 

 which these coloured borders could be seen continuously for any 

 length of time, so that, for example, a design in black lines appeared 

 as if drawn with red ink. The red border was explained by suppos- 

 ing a momentary sympathetic affection of the red nerve-fibres just 

 outside the boundary of the luminous patch projected upon the 

 retina. But, in some experiments, it was noticed that, when the 

 intensity of the illumination was increased beyond a certain limit, 

 the colour of the transient border was modified. Under very strong 

 illumination the colour of the border was greenish- blue ; sometimes 

 (as in Experiment II, p. 371) it was of two colours — greenish-blue 

 within and red without. It was with regard to the cause of these 

 greenish-blue and particoloured borders that I then considered it 

 preferable to " refrain from any speculation." 



It has now been ascertained, almost beyond doubt, that the 

 greenish-blue colour is due simply to the negative after-image of the 

 red, this after-image being in certain cases so much more conspicuous 

 than the red band which originates it, that the latter altogether 

 escapes notice. The experiments upon which this conclusion is 

 based gave results of a remarkable and unexpected character; there 

 is no record, so far as I am aware, that anything of the kind has been 

 previously noticed. 



It should be stated that the negative after-images here referred to 

 are of the kind which appear upon a white ground after the gaze 

 has been directed for a time upon a coloured object. The tints of 

 such after-images are generally said to be always complementary to 

 colours of the objects which excite them ; they are, however, rather 

 weak or impure, being largely diluted with white light. 



* ' Hoy. Soc. Proc./ 1896, vol. 60, p. 363. 



