LIGHT: COLOUR 303 



any pure spectrum colours ; but blue can be compounded (in 

 the manner described in 172) out of a narrow slit in the 

 green between the b and E lines, and a broad band in the 

 violet over the G line, rather to the violet end. Yellow can 

 be compounded in several ways. With the spectrum colours 

 it is done by rather a wide slit in the red, and a wider slit in 

 the green extending a little to each side of the b and E lines. 

 Another way is to project the spectrum, and from another 

 lantern, or other nozzle of a bi-unial, to throw on it the 

 image of a slit covered with red glass ; this slit being shifted 

 along the spectrum, will find a position over the green which 

 gives a good yellow. Thirdly, Lord Rayleigh superposed on a 

 film of gelatine stained deep blue with litmus (which cuts out 

 yellow and orange), one stained yellow with aurine (which 

 stops blue and violet). The two stop all but green and red, 

 and the result is yellow ; which is remarkable as yet another 

 result of blue and yellow films superposed. A better combina- 

 tion, however, also due to Lord Rayleigh, is a cell of litmus 

 solution, cutting out yellow and orange only as before, and 

 bichromate of potash, which stops blue ; the two allow to pass 

 the green and red, and give a remarkably good yellow, while 

 the colours themselves are so pure, that if a small round 

 aperture be focussed on the screen, the prism will disperse 

 this into two nearly sharp discs, one red and one green. 



174. Monochromatic Lights. These may be obtained fairly 

 pure in many ways. The most convenient yellow is from 

 combustion of sodium. A spirit lamp with a salted wick, or 

 a bead of fused salt in a platinum wire loop in a Bunsen 

 burner, will give a fair light, as will gas or hydrogen passed 

 through a saturated solution of salt. For a large light, for a 

 hall full of people, a quantity of tow with salt well rubbed 

 into it, soaked in methylated spirit, and burnt in a wire 

 basket over a vessel of water, gives a striking effect, all faces 

 appearing black and ghastly. A brighter light is obtained by 

 combustion of the actual metal, as presently mentioned. 



