LIGHT: COLOUR 289 



lantern another card, in which are cut two radial slots as in 

 fig. 155, so that the slots cross the nozzle and let flashes of 

 light through intermittently. Cover the nozzle itself with a 

 similar slot, taking care of course that the slots come just 

 where the rays cross from the objective. Then while the 

 Newtonian disc is rapidly rotated, let the other intercepting 

 disc also be rotated, at first slowly. The flashes by degrees, as 

 their speed increases, will not allow all the colours to mingle 

 their impressions during each, and so the disc will appear 

 coloured, till at last it will stand out almost distinctly in its 

 real colours 



From all these experiments it will be readily shown and as 

 readily understood, that for a ' pure ' spectrum we 

 must employ a narrow slit. The narrowest slit 

 give*} its own spectrum. That a broad slit gives 

 many overlapping spectra, is demonstrated by using, 

 instead of a single slit, a double slit arranged as 

 in fig. 156. At top and bottom there will be a 

 comparatively pure spectrum ; but over the greater 

 portion of the band the two will overlap, diluting 

 the colours by mixture ; the two slits here may be FIG. 156 

 regarded as the edges of a single wide slit. 



164. The Rainbow. The way in which a rainbow is 

 produced by the prismatic dispersion of each rain-drop was 

 shown by Antonio de Dominis, in an experiment easily 

 adapted to the lantern. A glass bulb blown on a small tube, 

 and from \\ to 2 inches in diameter, is filled either with 

 water, or with some other fluid of higher dispersion, or a 

 glass ball ground and polished would do, to represent our 

 spherical rain-drop. This bulb B is placed in a Bunsen 

 holder c, in front of the lantern, which must be turned 

 towards the spectators. The objective is removed, and the 

 parallel beam sent through a circular diaphragm with an 

 aperture the same diameter as the bulb; and surrounding 

 the nozzle, or at least the beam, is a screen of white card or 



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