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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-