1884.] on Rainbows. 4G7 



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, including coloured ones,* 

 have been tested by this arrangement, and very remarkable results 

 have been obtained. I will confine myself here to a reference to two 

 liquids, which commend themselves on account of their cheapness and 

 of the brilliancy of their efiects. 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 colour. With paraffin 

 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 analysed 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 colour-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 paraffin are both 

 more refractive than water, I thought it probable that in a mixed 

 shower of water and paraffin, 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 large and beautifully coloured water-wheel, the more 

 richly coloured circle of the turpentine makes its appearance. Or, 

 beginning with turpentine, and forming its concentrated iris ; on 

 turning on the water-spray, though to the eye the shower seems 

 absolutely homogeneous, its true character is instantly declared by 

 the flashing out of the larger concentric aqueous bow. The water 

 primary is accompanied by its secondary close at hand. Associated, 

 moreover, with all the bows, primary and secondary, are the super- 

 numeraries which belong to them ; and a more superb experimental 

 illustration of optical principles it would be hardly possible to witness. 

 It is not the less impressive because extracted from the simple com- 

 bination of a beam of light and a shower of rain. 



In the 'Philosophical Transactions' for 1835, the late Colonel 

 Sykes gave a vivid description of a circular solar rainbow, observed by 

 him in India, during periods when fogs and mists were prevalent in 

 the chasms of the Ghats of the Deccan. 



"It was during such periods that I had several opportunities of 

 witnessing that singular phenomenon, the circular rainbow, which, 



♦ Rose-aniline, dissolved in alcohol, produces a splendid bow with specially 

 broad supernumeraries. 



