218 THE RAINBOW 



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

 numeraries, extremely brilliant effects have been ob- 

 tained with this form of spray-producer. The quan- 

 tity of water called upon being much less than that 

 required by the rose, the fillet-and-disk instrument 

 produces less flooding of the locality where the experi- 

 ments 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 dura- 

 tion of the shower, will, when the shower is brief, be 

 evanescent. Hence, when quiet and continued contem- 

 plation of all the phenomena is desired, the observer must 

 make up his mind to brave the rain.* 



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, 



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

 expected, polarised from the drops. 



