the Light reflected by the Sky and by Plates of Glass. 137 



will be seen that the empirical curve gives results somewhat too 

 great for solar distances less than 60°, and too small for greater 

 distances ; but the deviation is so small, compared with the ac- 

 cidental errors, that we are justified in regarding the agreement 

 as complete within the limits of errors of observation. 



It will be noticed that no account is here taken of the points 

 of no polarization or neutral points of the sky ; but the polar- 

 ization is very slight for some distance from tbem, and hence is 

 not easily measured. They must be regarded as due to some 

 secondary disturbing cause, as refracted light, which alters the 

 general polarization of the sky but little. 



When the polarimeter is directed towards a polished coloured 

 plane surface, the two images assume different tints. One, 

 which contains the light polarized in the plane of incidence, or 

 A, is composed mainly of the light reflected specularly, and is 

 therefore white like the source of light. The image B contains 

 but little of the light reflected specularly, consisting principally of 

 the rays emitted by the body, and hence partaking of its colour. 

 The idea at once suggested itself, that testing the light of the 

 sky in this way might give a clue to the cause of its colour. 

 The experiment was tried several times with negative results, 

 the two images appearing of precisely the same blue tint. But 

 on the evening of July 15th, near sunset, when measuring the 

 polarization of a point near the northern horizon where the blue 

 colour was comparatively pale, a marked difference in the two 

 images was observable. The image A was found to be of a yel- 

 lowish brown, B of a greyish blue or violet tint. This observa- 

 tion has since been frequently repeated, and can, in fact, be 

 made almost any clear evening near sunset. Evidently w r e may 

 conclude from these colours that the true colour of the sky-par- 

 ticles is blue, a view quite in accordance with the observations 

 of Professor Cooke with the spectroscope, and of Professor Tyn- 

 dall on aqueous vapour in a state of formation. 



Observations were next made to test the results found above 

 for the light reflected and transmitted by several parallel sur- 

 faces of glass. To check the results which are given in figs 

 6-10, two, and in some cases three independent methods were 

 employed. To measure the polarization of the reflected ray, one 

 or more sheets of glass were laid on a piece of black velvet and 

 rendered horizontal with a spirit-level. The polarimeter was 

 then mounted a short distance from them, carefully levelled, 

 and then turned down so that the light should be reflected from 

 their surfaces. Its angle of depression would then equal the 

 complement of the angle of incidence. The line of junction of 

 the two images was then rendered vertical, and the polarization 

 measured in the usual way. The polarization of the sky, if 



