Contributions to the Physiology of Fision. 537 



There is another case of coloration which, I believe, has 

 not yet been noticed, and which admits of a similar expla- 

 nation. If a sheet of paper, with black characters, either 

 printed or written, be moved rapidly backwards and forwards at 

 the ordinary distance of distinct vision, the lines described by 

 the motion will appear accompanied by very evident colours, 

 the green and red obviously predominating. The experiment 

 succeeds better if the lines are far apart, and perpendicular 

 to the direction of the motion ; and is still more perfect if a 

 printed word be fixed at the extremity of a vibrating wire, (as 

 mentioned in the description of the Kaleidophone, in the Journal 

 of Science, N.S., No. xxiii. p. 344). This experiment indicates 

 that there is a faint production of colours at the limits of light 

 and darkness. 



From all the known facts, it may be inferred that luminous 

 impressions, continued on the *ame part of the retina, are 

 evanescent in proportion to their feebleness; and that there 

 are two means by which weak objects may be rendered con- 

 tinuously visible : 1st, by shifting their positions on the retina, 

 and 2ndly, by causing them to act intermittently on the same 

 points of the retina. 



Though these are obvious inferences from the collected 

 observations above stated, some of the facts separately pre- 

 sented might appear to admit of other explanations. Thus 

 Majendie and Desmoulins concluded, from the circumstances 

 noticed by them, ' that the sum of a certain number of impres- 

 sions on different points of the retina in a given time may ren- 

 der a body visible, which would not be so were the interval of 

 the impressions greater, or their number not sufficient.' This 

 explanation would answer only for a limited number of the 

 facts now brought together, and would exclude the experiments 

 of Pre'vost, where the image is periodically presented to the 

 same part of the retina. 



There are various other optical, or rather visual phenomena 

 which equally manifest the truth of the inferences above 

 drawn ; but as they are complicated with other circumstances 

 foreign to the present purpose, viz., illustrating the explanation 

 formerly given of the vascular figure of the retina, the{>fte$|- 

 deration of them will be deferred to a future occasioned* Htiw 



C. W. 



