64 Prof. Tyndall on Rainbows. 



bow, it follows that the higher the refractive index of the 

 liquid, the smaller will be the diameter of the circular bow 

 produced by its drops, Thus, the refractive index of spirit of 

 turpentine being greater than that of water, its circle ought 

 to be formed within that of water. Having produced the 

 extremely vivid bow of the turpentine shower, the water was 

 turned on. Its spray fell, mingled with the turpentine, and, 

 instantly, round the turpentine bow the larger water-bow was 

 swept through the darkness. The corresponding secondary 

 bows of both liquids were at the same time visible. For this 

 experiment water and petroleum are particularly handy liquids; 

 and I doubt whether among the phenomena of the laboratory 

 anything more beautiful is to be seen. 



Biot and Brewster proved the light of the ordinary rainbow 

 to be polarized. As might be expected, in the artificial bow 

 the light is also polarized. It is transmitted when the long 

 diagonal of a nicol is tangent to the bow ; it is quenched 

 when the short diagonal is in this position. 



I have mentioned the " doorways " in the Alps and at Hind 

 Head as if they contributed to the effect. They did so by 

 limiting the portion of fog illuminated, thus enabling the 

 eye to project the bow upon the dark surrounding space. 

 This action of the eye is physiologically interesting. When 

 a camera and lens are used, the beam from which the eye re- 

 ceives the rays producing the bow may not be more than 

 a foot in width ; while it is easy to obtain from these rays a 

 circular bow with an apparent diameter of fifteen or twenty 

 feet. By varying the position of the head, the position and 

 magnitude of the bow is caused to undergo corresponding 

 alterations. 



I need not say that caution is necessary in dealing with in- 

 flammable liquids. Explosive mixtures might readily be pro- 

 duced. It is therefore well to isolate by a plate of glass or 

 otherwise the room in which the light is placed from that in 

 which the spray is produced ; taking care, moreover, to avoid 

 in the latter the ignition of matches, or the introduction of 

 any other light. 



The action of mineral solutions is well illustrated by chlo- 

 ride of ammonium. Forming the water-bow first, and then 

 permitting the spray of the solution to descend, the liquids 

 do not coalesce to form a weaker solution. Within the water- 

 bow, but not much within it, the smaller bow of the solution 

 appears. 



Royal Institution, 

 December 14th, 18S3. 



