THE BOOK OF THE LANTERN. 185 



will have prismatic colours at opposite ends, but the entire 

 central portion will be white. To show that the colours of 

 the spectrum pass over every portion of the path of the 

 light, as indicated by the band, the prism may be rocked 

 very slowly. 



"By inserting four screw hooks in a vertical support, and 

 stretching the bands over the hooks, the prism is adapted 

 for use with a lantern. The light emerging from the 

 lantern must pass through a narrow slit to secure a per- 

 fect spectrum, and between the screen and the prism 

 should be placed another screen with an oblong aper- 

 ture, which will allow all of the band of light to appear 

 upon the screen, with the exception of the coloured 

 extremities. With the prism supported in this way, it is 

 an easy matter to turn it slowly back and forth, showing 

 on the screen the moving spectrum, which, with the more 

 rapid movement, produces the pure white band of light." 



The recomposition of light can be well shown in the 

 way just described ; but perhaps a more ready and effec- 

 tive, if not quite so scientific, a method is to use a coloured 

 disc, fitted as a lantern-slide, with a revolving arrangement 

 similar to that used for chromotropes. 



Newton's disc, as it is called, consists of all the colours 

 of the spectrum, painted in transparent colours, in their 

 right proportions, upon a revolving disc, and as this 

 disc is rapidly turned in the lantern, the various 

 colours projected upon the screen in front mingle together 

 on the retina, and the general effect is that of white 

 light. It may happen that a lecturer may touch 

 upon the study of spectra without wishing to burden him- 



