184 THE BOOK OF THE LANTERN. 



placed upon a stand that they can each be turned in any 

 required direction. The spectrum is allowed to fall upon 

 this system of mirrors, and each one is so turned upon its 

 axis that the particular colour which it reflects is thrown 

 upon one spot. The collective images of the various 

 colours then appears as a white disc. 



The methods thus detailed are all good, but cannot 

 readily be applied to the lantern. A way of demonstrating 

 the recomposition of light with that instrument has 

 recently been published in America, by Mr. G. M. Hop- 

 kins, and the following remarks are borrowed from him. 

 After detailing the various known methods of recomposing 

 light, he says : "Besides these methods, the spectrum has 

 been recombined by whirling or rocking a prism ; the move- 

 ment of the spectrum being so rapid as to be beyond the 

 power of the eye to follow, the retina receiving the impres- 

 sion merely as a band of white light, the colours being 

 united by the superposing of the rapidly succeeding 

 impressions, which are retained for an appreciable length 

 of time. The engraving shows a device to be used in 

 place of the ordinary rocking prism. It is perfectly 

 simple, and involves no mechanism. It consists of an 

 inexpensive prism, having attached to a knob on either 

 end a rubber band. In the present ease the bands are 

 attached by making in each a short slit, and insert- 

 ing the knobs of the prism in the slit. The rubber 

 bands can be held by inserting two fingers in each and 

 drawing them taut. The prism can then be held in a 

 beam of sunlight, and with one finger the prism is given 

 an oscillating motion. The band of light thus elongated 



