212 



SIR DAVID BREWSTER 



were not difficult to make when the sky was clear and blue ; but in studying 

 the part of the atmosphere between the sun and the horizon, I was perplexed 

 beyond measure with the feeble and uncertain indications of the polariscope. The 

 sky between the sun and the horizon is always the most impure portion of it ; 

 and the flood of light streaming from the sun unfits the eye for detecting faint 

 traces of colour. Theoretical considerations had led me to believe that a neutral 

 point should be found between the sun and the horizon, and certain indications 

 of the polariscope, at places around its probable locality, entitled me to infer that 

 it did exist ; but, as an inference was not satisfactory, I watched every favourable 

 state of the sky, in the hope of obtaining a more direct result. 



An opportunity of doing this at last presented itself to me on the 28th 

 February 1842, when the sun was in the meridian, with an altitude of 22°. The 

 spot beneath the sun was fortunately vibsile from the end of a long dark passage 

 running north and south, and having concealed the sun himself, and every part 

 around him except the probable position of the spot, I obtained a most distinct 

 view of the new neutral point, situated about 1 5° or 16° below the sun. After com- 

 municating this discovery to M. Babinet early in 1845, he made several ineffec- 

 tual attempts to confirm it, and it was not till the 23d July 1846 that he suc- 

 ceeded in obtaining a distinct view of this neutral point. Following the usual 

 practice, the French have given to this spot the name of Brewster's neutral point. 



MAX. fOLAR 



BRZV/STEfCi 



ANTISO LAR 

 POINT 



Fis. 1. 



With these three elements of atmospherical polarisation, shown in Fig. 1, 

 I shall now proceed to determine their position in relation to the sun and the 

 antisolar point, and ascertain the changes which they undergo from variations 

 in the optical and meteorological state of the atmosphere. 



