428 



ELEMENTARY PHYSIOLOGY 



LESS. 



that if any two luininoiis impressions are separated by 

 a less interval, they are not distinguished from one 

 another. 



For this reason a " Catherine-wheel," or a lighted stick 

 turned round vei-y rapidly by the 

 hand, appears as a circle of fire ; 

 and the spokes of a coach wheel at 

 speed are not separately visible, 

 but only appear as a sort of opacity, 

 or film, within the tyre of the 

 wheel. 



The cinematograph is based 

 upon this fact. A series of in- 

 stantaneous photographs of some 

 object in motion, taken at the rate 

 of many per second, is printed on 

 a long transparent film of celluloid. 

 The film is then passed through a 

 magic-lantern at such a rate that 

 not less than ten of the consecutive 

 photographs are projected on to 

 the screen in each second. At 

 this rate, the impression produced 

 by one photograph has not had 

 time to die out before the next 

 one produces its slightly different 

 later effect. The result is that 

 the consecutive pictures on the 

 screen blend in succession one 

 into the other and so reproduce the 

 appearance of the original moving 

 object. 



11. Sensations of Light Produced without the 

 Action of Light. — The sensation of light may be excited 

 by other causes than the impact of the vibrations of the 

 luminiferous ether upon the retina. Thus, an electric 



