174 THE HUMAN BODY. 



displacement of the spots proves that it is not permanent. 

 We know also that this partial insensibility may be induced 

 by a brilliant light, and particularly by the rays of the sun. 

 It is an experiment we all make involuntarily, and which will 

 be discussed in another place. 



Persistence of retinal impressions. The impressions made 

 by the luminous rays remain for a certain time, and are then 

 gradually effaced; it is plain then, that if the action is repro- 

 duced at shorter intervals than the duration of the impres- 

 sions, the brain perceives, not a series of sensations, but a 

 continuous one. Thus in the rapid rotary movement of a 

 burning coal, the eye perceives only a luminous circle, and 

 when a wheel revolves rapidly, the spokes seem to approach 

 each other and form a continuous surface. The impression 

 of colour persists as well as of form ; and if we cause a circle 

 divided into party-coloured sections to revolve rapidly, they 

 produce the sensation formed by a blending of them together; 

 reel and blue, for example, look like violet, and a great 

 variety of different shades produce an impression as of gray. 

 According to M. Plateau, the duration of impressions on the 

 retina is about half a second. 



This persistence of impressions has given rise to the con- 

 struction of an apparatus, which is at the same time an 

 object of amusement and a curious philosophical instrument. 

 Such is, for example, the phcnakistiscope. It was upon the 

 same principle that the beautiful experiments were founded, 

 by the aid of which Wheatstone measured the duration of 

 lightning flashes. 



Accidental images. We may compare to a certain extent 

 the action of light on the retina to that of pressure on an 

 elastic surface. When the rays of any colour strike the 

 retina, it resists the impulse of the luminous wave, and 

 strives to regain a state of repose. When the action of light 

 abruptly ceases, as when we close the eyes, for example, 

 after a very short time, which is measured by the duration of 

 the impression produced, the retina returns to its normal 

 state by a reaction which is more energetic in proportion to 

 the length or duration of the action. It passes by a sort of 

 oscillation from the condition in which it was placed by the 



