﻿its Polarization and Colour. 275 



wave-length of light, and have applied the results to explain the 

 phenomena presented hy the sky. Another theory has been 

 given by Clausius, who attributes the light of the sky to reflec- 

 tion from water-bubbles, and has developed his views at length 

 in a series of papers in Poggendorff's Annalen and Crelle's 

 Journal*. 



Starting from the ordinary laws of reflection and refraction, 

 he has no difficulty in showing that, were the atmosphere charged 

 with globes of water in sufficient quantity to send us the light 

 which we actually receive, a star instead of appearing as a point 

 would be dilated into a disk of considerable magnitude. But the 

 requirements of the case are satisfied if we suppose the spheres 

 hollow, like bubbles ; for then, on account of the parallelism of 

 the surfaces, but little effect is produced by refraction on a wave 

 of light. At the same time, if the film be sufficiently thin, the 

 light reflected from it will be the blue of the first order, and so 

 the colour of the sky is apparently accounted for. 



Apart from the difficulty of seeing how such bubbles could be 

 formed, there is a formidable objection to this theory, mentioned 

 by Briicke (Pogg. Ann. vol. lxxxviii. p. 363) — that the blue of 

 the sky is a much better colour than the blue of the first order. 

 That it is so appears clearly from the measurements quoted in 

 the February Number, and from the theoretical composition of 

 the blue of the first order f. Nor can we escape from this diffi- 

 culty by supposing, with Briicke, that the greater part of the 

 light from the sky has been reflected more than once. 



Briicke also brings forward an experiment of great importance 

 when he shows that mastic precipitated from an alcoholic solu- 

 tion scatters light of a blue tint. He remarks that it is impos- 

 sible to suppose that the particles of mastic are in the form of 

 bubbles. 



In his last utterance on this subject J, Clausius replies to the 

 objections urged by Briicke and others against his theory, and 

 shows that, if the illumination of the sky is due to thin plates at 

 all, those thin plates must be in the form of bubbles. While ad- 



* Pogg. Ann. vols, lxxii. lxxvi. lxxxviii. Crelle, vols, xxxiv. xxxvi. 



t I find that I omitted to explain why it is that the light dispersed from 

 small particles is of so much richer a hue than that reflected from vervthin 

 films. In the latter case the reflected wave may be regarded as the sum 

 of the disturbances originating in the elementary parts of the film, and 

 these elementary parts may be assimilated to the small particles of the 

 former supposition. The integration is best effected by dividing the sur- 

 face into the zones of Huyghens ; and it is proved in works on physical op- 

 tics that the total effect is just half of that due to the first zone. Now the 

 zones of Huyghens vary as the wave-length ; and thus it appears that in the 

 integration the long waves gain an advantage which diminishes the original 

 preponderance of their quicker-timed rivals. 



X Pogg. Ann. vol. lxxxviii. p. 543, 



