62 Prof. Tyndall on Rainbows. 



and condensing above to droplets of a similar kind, was blown 

 violently into the atmosphere. A combination of fog and drizzle 

 similar to that observed in the Alps was thus obtained. A 

 few trials sufficed to bring the luminous circle into view, and 

 by a little practice its light was greatly enhanced. 



In these experiments the light passed from one room 

 through a doorway into another room, in which the boiler 

 was placed. The naked limelight and the naked electric light 

 produced, as might be expected, effects more vivid than those 

 of an ordinary oil- or gas-lamp. But the maximum brilliancy 

 was obtained when the electric light was placed in a camera, 

 with a condensing lens in front, the beam being rendered suf- 

 ficiently divergent to leave a luminous area round the shadow 

 of the head cast upon a distant black screen. The circular 

 bow was then extremely vivid, its convex circumference being 

 distinctly red, and its concave circumference distinctly blue. 

 Outside the primary, the corresponding secondary bow was 

 seen, with the colours of the primary reversed. By mea- 

 surement with a theodolite, I found the angle subtended by 

 the radius of the most luminous primary circle to be 42°, its 

 position being therefore identical with that of the rainbow. 



Showers of liquids other than water were next examined, 

 various methods of forming spray being more or less success- 

 fully tried. Probably different liquids will require different 

 spray-producers to bring out their best effects. For the pro- 

 duction of water-spray, the most convenient arrangement was 

 found to be that employed to moisten the air in the Houses of 

 Parliament, and, I believe, employed also in the washing of 

 gas. It is a modification of a well-known experiment of 

 Savart. From a narrow orifice a jet of water, under high 

 pressure, is caused to impinge upon a small metallic disk : 

 the liquid spreads laterally, and breaks up into extremely fine 

 droplets. My assistant reminded me of this spray-producer ; 

 and Mr. Tylor was good enough to borrow the little instru- 

 ment for me from Mr. Pirn. Fixed near the ceiling of our 

 laboratory, and connected with the water-pipes of the Insti- 

 tution, on turning on a cock a very fine shower falls down- 

 wards. The electric light being placed, as stated, in an ad- 

 jacent room, and the lens being so arranged that the beam 

 embraces the head leaving an aureole round its shadow, effects 

 of great beauty are obtained. The circular primary bow 

 comes out with extreme vividness, and may be looked at for 

 any length of time*. The secondary bow is also obtained as 

 a complete circle surrounding the primary. 



The manner in which the spray is formed renders the drops 

 * Waterproof clothing is desirable in these experiments. 



