182 THE BOOK OF THE LANTERX. 



speed of either can be varied at will by the operator. The 

 effect upon the screen is most curious, for it seems to be 

 covered with a lacework of geometrical patterns which 

 constantly change their form. 



A very favourite experiment with the lantern, but one 

 which it is by no means easy to perform, is the decomposi- 

 tion of light by means of a prism. For the most perfect 

 effects the electric light is necessary, but as this is beyond 

 the reach of most of us, at any rate, for the present, we 

 must be content with what can be done with the ordinary 

 limelight. The simplest way of showing the spectrum 

 with the lantern is to remove the objective and to place in 

 the lantern stage a card with a slit in it, as 

 shown in the cut (fig. 47). This slit should be 

 about an inch in length, and not more than 

 one-twentieth of an inch in breadth. The card 

 should be placed on the stage of the lantern in a 

 IIG. 47. h or i z ontal position and focussed upon the screen 

 in front. A prism is then brought into the path of the 

 slice of light thus formed, 

 and it will be so far bent 

 aside as to exhibit the. 

 colours of the spectrum on 

 the ceiling of the room 

 (fig. 48). FIG. 48. 



The prism will require a little turning about before this 

 result is arrived at. But at the best this method of show- 

 ing the spectrum is but a makeshift one ; it presents, 

 however, an easy method of demonstrating the decom- 

 position of white light. A preferable mode is to use a 



