LECT. XX. OCULAR SPECTRA. 385 



of the preceding until the following one takes place. There 

 results from this persistence, an effect similar to that which 

 would be produced by the object in motion. 



Plateau, who long studied this subject, discovered that, 

 in order to produce a complete impression, the light ought 

 to act for a certain time; that the total duration of the im- 

 pression is the same for all the colours, and may be ap- 

 proximatively estimated at //< 34; that the interval of time, 

 during which the impression preserves the same intensity, 

 is as much more considerable as the light is more mode- 

 rated ; that it is different for rays of different colours; for 

 instance, it is longer for the blue than for the red and white 

 light ; and that, in fact, the total duration of the impressions 

 is so much the more persistent as the light is the more in- 

 tense and its action less prolonged. A cannon-ball does 

 not leave the impression of its passage like a shooting star, 

 because the intensity of its light is less. 



Ocular Spectra. Besides the persistence of impression 

 received by the retina, other phenomena occur to us by no 

 means less curious, when we have an object fixed for a cer- 

 tain time. Look at a. disc of any colour placed in the mid- 

 dle of a piece of black card, after having fixed your eyes on 

 it for a certain time remove them rapidly to a white ground, 

 or cover them with a cloth, and you will then apparently 

 see a disc, in form similar to the first, but having a colour 

 complementary to it. Thus, if the disc be red, the image 

 will be green ; if yellow, it will appear violet ; and if white, 

 it will look gray. These apparent sensations have received 

 the name of accidental colours. Plateau seems to have de- 

 monstrated that these images cease, after having presented 

 a singular series of phenomena ; thus it appeared, that after 

 a certain time they vanish, and give place to an image 

 which has really the colour of the object ; this second dis- 

 appears, and that of the complementary colour returns. 

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