98 The Persistence of Luminous Impressions. 



THE PEBSISTENCE OF LUMINOUS IMPEESSIONS. 



BY THE ABBE LABOKDE. 



The following paper is translated from Gomptes Bendus,. 

 16th July, 1866 :— 



" When a luminous point strikes the eye and suddenly 

 disappears, the sensation which it produces does not become 

 immediately extinct, and according to the researches of some 

 physicists, it remains for about one-third of a second, from 

 whence arises all the phenomena known and explained under 

 the title of persistent luminous impressions. 



" It occurred to me to inquire whether in white light all 

 the component colours had the same degree of persistence, 

 and to study this question I submitted- the sensation of light 

 to an experiment which revealed to me a very curious fact, 

 which seems to demonstrate that in white light the most 

 refrangible colours are more persistent than the others, and 

 that they act in advance of the others, so that the organ of 

 vision decomposes white light by dispersing its colours in 

 different times, just as the prism decomposes it by dispersing 

 in different places. 



" I make this experiment : the light of the sun is received 

 upon a mirror which throws it horizontally on to a slit made 

 in the shutter of a dark chamber. This slit should be three- 

 millimetres* wide and six long. Close to this slit, inside the 

 chamber, a disk of metal is placed, on the margin of which, 

 similar slits are made, with wide interspaces between them. 

 A clockwork movement turns the disk, and a clamp, which the- 

 observer can operate upon at a distance, allows the revolution 

 to take place with greater or less velocity. In the line of the 

 light rays, at a distance of about a metre, a screen of ground 

 glass is placed, behind which the modifications of the light 

 can be observed when the disk is in motion. At first the 

 light is made to appear or disappear slowly, and is found 

 uniformly white ; but when the successive appearances are 

 made to take place more rapidly, the margins begin to be 

 tinted, and by progressively increasing the velocity, the 

 surface of the image becomes successively affected by blue, 

 green, rose, white, green, blue. After this last blue, increasing 

 velocities restore the whiteness of the light. 



" The whole of the phenomena depend, as will be seen, 

 on the varying periods occupied by the revolutions of the 

 disk." 



* In Darling's Metric Talles we find 3 min — O'llS of an inch. 



