164 



LETTERS ON NATURAL MAGIC. 



It would lead us into too wide a field were we to detail 

 the immense variety of resources which the science of optics 

 furnishes for such exhibitions. One of these, however, is 

 too useful to be passed without notice. If we interpose a 

 prism with a small refracting angle between the image a b, 

 Fig. 7, and the lens L L, the part of the figure immediately 

 opposite to the prism will be as it were detached from the 

 figure, and will be exhibited separately on the screen P Q. 

 Let us suppose that this part is the head of the figure. 

 It may be detached vertically, or lifted from the body as 

 if it were cut off, or it may be detached downwards and 

 placed on the breast as if the figure were deformed. In 

 detaching the head vertically or laterally, an opaque screen 

 must be applied to prevent any part of the head from 

 being seen by rays which do not pass through the prism ; 

 but this and other practical details will soon occur to those 

 who put the method to an experimental trial. The applica- 

 tion of the prism is shown in Fig. 8, where a b is the 



Fig. S. 



inverted image formed by a concave mirror, A B C a 

 prism with a small refracting angle B C A, placed between 

 a b and the lens L L, s a small opaque screen, and A B 

 the figure with its head detached. A hand may be made 

 to grasp the hair of the head, and the aspect of death may 

 be given to it, as if it had been newly cut off. Such a 

 representation could be easily made, and the effect upon 



