Prof. Tyndall on Rainbows. 63 



which constitute the shower of sensibly uniform size. The con- 

 sequence is that within the primary we have a surprising dis- 

 play of supernumerary bows*. I have counted six concentric 

 circles thus formed by interference within the primary. Round 

 the convex circumference of the secondary bow a series of paler 

 supernumeraries is also to be seen. By properly managing 

 the illuminating-beam these effects may be rendered extremely 

 sharp and beautiful. The light from a Siemens dynamo 

 machine produces very fine effects; while the light of the vol- 

 taic battery is particularly suitable when steadiness, sharp- 

 ness, and vividness are sought to be combined. 



To obtain the spray of liquids other than water, the following 

 arrangement is made : — A cylindrical iron bottle, used for the 

 storing of liquefied carbonic acid, is filled with the liquid to 

 be experimented on. Through a brass cap, which can be 

 firmly screwed on to the top of the bottle, passes a metal tube 

 t which ends close to the bottom ; to the outside end of this 

 tube is attached the spray-producer. A second tube passes 

 also through the cap, and ends immediately underneath it. 

 This tube is united by a long and flexible compo-tube to a larger 

 iron bottle containing compressed oxygen, hydrogen, or air. 

 Hoisting the small bottle to a convenient height, the tap of 

 the larger bottle is carefully opened : the gas passes through 

 the compo-tube to the smaller bottle, and exerts its pressure 

 upon the surface of the liquid therein contained. The liquid 

 is forced upwards through the tube t, and passes to the spray- 

 producer, from which it falls in a fine rain. A great many 

 liquids have been tested by this arrangement, and very re- 

 markable effects have been produced. 



Spirit of turpentine, forced thus from the iron bottle, and 

 caused to fall in a fine shower, produces a circular bow of ex- 

 traordinary intensity. With paraffin oil a similar effect is 

 obtained. I have also tried bisulphide of carbon, tetrachloride 

 of carbon, chloroform, benzol, pentane, and other liquids, in- 

 cluding various mineral solutions ; but to those who wish to 

 make the experiment in a cheap, an easy, and an effectual 

 way, I would recommend the sprays of spirit of turpentine 

 and of petroleum. 



I asked myself whether, in the case of a composite shower 

 (that is to say, a shower produced by the spray of two or more 

 different liquids) the beam would be able to analyse the 

 shower and, by the production of two or more distinct pri- 

 maries, reveal its constituents. This was found to be the case. 

 From the principles which underlie the formation of the rain- 



* Professor Stokes reminds me that the angular smallness of the source 

 of light also contributes to this effect. 



