THE PRAXINOSCOPE. 



123 



arrangement. In the Prxxinoscope* (a name given by the inventor, Mr. 

 Reynaud, to this new apparatus), the substitution of one object for another 

 is accomplished without interruption in the vision, or solution of con- 

 tinuity, and consequently without a sensible reduction of light ; in a 

 word, the eye beholds continuously an image which, nevertheless, is inces- 

 santly changing before it. The result was obtained in this manner. Hav- 

 ing sought unsuccessfully by mechanical means to substitute one object for 

 another without interrupting the continuity of the spectacle, the inventor was 

 seized with the idea of producing this substitution, not with the objects 



Fig. 125. Plateau's Phenokistoscope. 



themselves, but with their virtual images. He then contrived the arrange- 

 ment which we will now describe. A plane mirror, AB (fig. 129), is placed at 

 a certain distance from an object, C D, and the virtual image will be seen at 

 C'D'. If we then turn the plane mirror and object towards the point, O, 

 letting B E and D F be their new positions, the image will be at c'V. 

 Its axis> O, will not be displaced. In the positions, A B and C D, first occupied 

 by the plane mirror and the object, we now place another mirror and object. 

 Let us imagine the eye placed at M. Half of the first object will be seen at 

 O D", and half of the second at O c'. If we continue the rotation of the 

 instrument, we shall soon have mirror No. 2 at T T', and object No. 2 at S S'. 

 At the same moment the image of object No. 2 will be seen entirely at 

 C'"D'' . Mirror No. 2 and its object will soon after be at B E and D F. If we 

 then imagine another mirror and its corresponding object at A B and C D, 

 the same succession of phenomena will be reproduced. This experiment 

 therefore shows that a series of objects placed on the perimeter of a polygon 



* From praxis, action, and skopein, to show. 



