644: THE SENSES. 



disk, with a perforation at one spot near its edge, through which another 

 disk is visible, placed immediately behind the first, and capable of re- 

 volving rapidly while the first remains stationary. Upon the second disk 

 is a circle of pictures representing the same figure in different positions ; 

 and when, by its revolution, these pictures are made to pass in quick 

 succession across the opening of the disk in front, they present the ar> 

 pearance of a single figure in rapid motion. The interval between the 

 perception by the eye of successive pictures is too short to be observed, 

 and the same object appears to take successively the different positions 

 in which it is represented. 



Duration of a Luminous Impulse required for the Perception of 

 Visual Impressions. This point has been investigated by Rood 1 by 

 means of the light of an electric spark obtained from an induction coil 

 connected by its terminal wires with the inner and outer surfaces of a 

 Leyden jar. On breaking the primary current a discharge takes place 

 between the electrodes, which is of exceedingly short duration. This 

 duration was measured by Prof. Rood with the aid of a mirror revolv- 

 ing upon its transverse axis, by which the light of the electric spark 

 was thrown upon a plate of glass, where it could be examined by the 

 naked eye, or with a magnifying eye-piece, as in Fig. 208. 



The light emanating from the spark S, was received by an achromatic 

 lens L, of nine inches focal length. It then fell upon a plane mirror 

 revolving with a uniform velocity of 340 times per second, and, after 

 reflection by the mirror, was brought to a focus upon a glass plate G, 

 where it could be examined by the telescope eye-piece E, magnifying 



ten diameters. From the known 

 rate of revolution of the mirror, and 

 its distance from the glass plate G, 

 the necessary rate of movement of a 

 reflected beam upon the plate was 

 determined. If the spark, used in 

 these experiments, lasted long 

 enough for its reflected image to 

 move over an appreciable distance, 

 this image would appear to the eye 

 to be drawn out in the direction of 

 the movement, owing to the persist- 

 ence of its visual impression as de- 

 Apr AR A TUB for measuring the dura- scribed above. But with the mirror 



tion of an electric spark -S Position of revo l v i ng at this Speed no SUCh de- 

 the spark. L. Achromatic lens. M. Re- 

 volving mirror. G. Glass plate for receiv- formation was perceptible, the spark 

 ing the image of the spark. E. Telescope i ma g e appearing of precisely the 



same form as if the mirror were 



stationary; showing that the duration of the light could not be greater 

 than .000002 ( 500 'ooo) of a second. 



1 The American Journal of Science and Arts. New Haven, September, 1871. 



