﻿278 On the Light from the Sky, its Polarization and Colour. 



sky. At any rate it is difficult to imagine particles of water 

 smaller than the wave-length endowed with any stability. These 

 difficulties might perhaps be got over if there were any strong 

 argument in favour of the water-particles ; but of the existence 

 of such I am not aware. Every one knows that a blue haze 

 evidently akin to the azure of the sky obliterates the details and 

 modifies the colour of a distant mountain ; and this, when it 

 occurs on a hot day, cannot possibly be attributed to aqueous 

 particles. On the face of it, there is no reason for supposing 

 that near the earth's surface the foreign matter is of one kind 

 and at a great altitude another. If it were at all probable that 

 the particles are all of one kind, it seems to me that a strong 

 case might be made out for common salt. Be this as it may, 

 the optical phenomena can give us no clue. 



The apparatus by means of which the comparison was made 

 between sky light and that of the sun diffused through white 

 paper, was originally arranged for measurements of the absolute 

 absorption of coloured fluids for the various rays of the spectrum, 

 and had been applied rather extensively in experiments having 

 that object. In the shutter of a darkened room were placed two 

 slits in the same vertical line, each about three inches long, and 

 a foot apart. At the other end of the room was an arrangement 

 of prisms and lenses for producing a pure spectrum on a screen 

 in the ordinary way. At first only one prism was used ; but I 

 soon introduced another; and the number might probably be 

 further increased with advantage. It is even more important to 

 have a great dispersion in these experiments than in the ordinary 

 spectroscope. Two spectra would thus be thrown on the screen 

 one over the other, but by means of a very obtuse-angled prism 

 situated in front of the dispersion-prisms they are brought 

 together so as exactly to overlap. The double spectrum thus 

 formed passes through a horizontal slit in the screen placed so 

 as to receive it. Close behind is an opaque card carrying a 

 small vertical slit, which can be slid along so as to allow any 

 desired part of the spectrum to pass through. At the beginning 

 and end of a set of experiments the card is removed, and the 

 principal fixed lines are observed through an eyepiece and re- 

 ferred to a scale situated just over the horizontal aperture. 



When the experimenter looks through the eye-slit in the di- 

 rection of the lens, he sees the two parts of the obtuse prism 

 illuminated with light, in each case homogeneous and, if the 

 adjustments are properly made, belonging to the same part of the 

 spectrum. By varying the breadths of the original slits, the two 

 parts of the field may be made equally bright ; and when the 

 match is attained, the breadths are inversely proportional to the 

 richness of the lights behind them in the homogeneous ray under 



