ON RAINBOWS. 671 



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.* 



In one important particular the spray-producer last described com- 

 mends 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-producer. A second tube passes also through 

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

 which is long and flexible, is connected with a larger iron bottle, con- 

 taining compressed air. Hoisting the small bottle to a convenient 

 height, the tap of the larger bottle is carefully opened, the air passes 

 through the flexible tube to the smaller bottle, exerts its pressure upon 

 the surface of the liquid therein contained, drives it up the other 

 tube, and causes it to impinge with any required degree of force 

 against the disk of the spray-producer. From this it falls in a fine 

 rain. A great many liquids have been tested by this arrangement, 

 and very remarkable results have been obtained. I will confine my- 

 self here to a reference to two liquids, which commend themselves on 

 account of their cheapness and of the brilliancy of their effects. Spirit 

 of turpentine, forced from the iron bottle, and caused to fall in a fine 

 shower, produces a circular bow of extraordinary intensity and depth 

 of color. With paraffine-oil or petroleum a similar effect is obtained. 



Spectrum analysis, as generally understood, occupies itself with 

 atomic, or molecular, action, but physical spectrum analysis may be 

 brought to bear upon our falling showers. I asked myself whether a 

 composite shower that is to say, one produced by the mingled spray 

 of two or more liquids could not be analyzed and made to declare its 

 constituents by the production of the circular rainbows proper to the 

 respective liquids. This was found to be the case. In the ordinary 

 rainbow the narrowest color-band is produced by its most refrangible 

 light. In general, the greater the refraction, the smaller will be the 

 bow. Now, as spirit of turpentine and paraffine are both more refrac- 

 tive than water, I thought it probable that in a mixed shower of water 

 and paraffine, or water and turpentine, the smaller and more luminous 

 circle of the latter ought to be seen within the larger circle of the 

 former. The result was exactly in accordance with this anticipation. 

 Beginning with water, and producing its two bows, and then allowing 

 the turpentine to shower down and mingle with the water, within the 



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

 from the drops. 



