218 THE EAINBOW 



quired. In regard to primaries, secondaries, and super- 

 numeraries, extremely brilliant effects have been 

 obtained with this form of spray - producer. The 

 quantity of water called upon being much less than 

 that required by the rose, the fillet-and-disk instru- 

 ment produces less flooding of the locality where the 

 experiments are made. In this latter respect, the 

 steam-boiler spray is particularly handy. A puff of two 

 seconds' duration suffices to bring out the bows, the 

 subsequent shower being so light as to render the use 

 of waterproof clothing unnecessary. In other cases, 

 the inconvenience of flooding may be avoided to a 

 great extent by turning on the spray for a short time 

 only, and then cutting off the supply of water. The 

 vision of the bow being, however, proportionate to the 

 duration of the shower, will, when the shower is brief, 

 be evanescent. Hence, when quiet and continued 

 contemplation of all the phenomena is desired, the 

 observer must make up his mind to brave the rain. 1 



In one important particular the spray-producer last 

 described commends itself to our attention. With it 

 we can operate on substances more costly than water, 

 and obtain rainbows from liquids of the most various 

 refractive indices. To extend the field of experiment 

 in this direction, the following arrangement has been 

 devised : A strong cylindrical iron bottle, wholly or 

 partly filled with the liquid to be experimented on, is 

 tightly closed by a brass cap. Through the cap passes 

 a metal tube, soldered air-tight where it crosses the 

 cap, and ending near the bottom of the iron bottle. To 

 the free end of this tube is attached the spray -pro- 

 ducer. A second tube passes also through the cap, but 

 ends above the surface of the liquid. This second tube, 



1 The rays which form the artificial bow emerge, as might be 

 expected, polarised from the drops. 



