﻿The Hon. J. W. Strutt on the Light from the Sky. 107 



filtrate evaporated to dryness, gently ignited, and the mixed 

 chlorides weighed. 



The separation of potassa and soda was effected in the usual 

 way by means of chloride of platinum. 



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

 By theRon. J. W. Strutt, Fellow of Trinity College, Cambridge*. 



IT is now, I believe, generally admitted that the light which 

 we receive from the clear sky is due in one way or another 

 to small suspended particles which divert the light from its re- 

 gular course. On this point the experiments of Tyndall with 

 precipitated clouds seem quite decisive. Whenever the particles 

 of the foreign matter are sufficiently fine, the light emitted late- 

 rally is blue in colour, and, in a direction perpendicular to that 

 of the incident beam, is completely polarized. 



About the colour there is no prima facie difficulty; for as 

 soon as the question is raised, it is seen that the standard of 

 linear dimension, with reference to which the particles are called 

 small, is the wave-length of light, and that a given set of par- 

 ticles would (on any conceivable view as to their mode of action) 

 produce a continually increasing disturbance as we pass along 

 the spectrum towards the more refrangible end; and there 

 seems no reason why the colour of the compound light thus 

 scattered laterally should not agree with that of the sky. 



On the other hand, the direction of polarization (perpendicular 

 to the path of the primary light) seems to have been felt as a 

 difficulty. Tyndall says, u . . . . the polarization of the beam by 

 the incipient cloud has thus far proved itself to be absolutely inde- 

 pendent of the polarizing- angle. The law of Brewster does not 

 apply to matter in this condition ; and it rests with the undula- 

 tory theory to explain why. Whenever the precipitated particles 

 are sufficiently fine, no matter what the substance forming the 

 particles may be, the direction of maximum polarization is at 

 right angles to the illuminating beam, the polarizing angle for 

 matter in this condition being invariably 45°. This I consider 

 to be a point of capital importance with reference to the present 

 question" t. As to the importance there will not be two opi- 

 nions; but I venture to think that the difficulty is entirely 

 imaginary, and is caused mainly by misuse of the word reflection. 

 Of course there is nothing in the etymology of reflection or re- 

 fraction to forbid their application in this sense ; but the words 

 have acquired technical meanings, and become associated with 

 certain well-known laws called after them. Now a moment's 

 consideration of the principles according to which reflection and 

 * Communicated by the Author. 

 t Phil. Mag. S.4. vol. xxxvii. p. 388. 



