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VII. 



EXPERIMENTS AND INVESTIGATIONS ON LIGHT AND 

 COLOUES. 



THE optical inquiries of which I am about to give an account, 

 were conducted at this place in the months of November and 

 December 1848, and continued in autumn 1849 at Brougham, 

 where the sun proved of course much less favourable than in 

 Provence : they were further prosecuted in October. I had 

 thus an opportunity of carefully reconsidering the conclu- 

 sions at which I had originally arrived ; of subjecting them 

 first to analytical investigation, and afterwards to repetition 

 and variation of the experiments ; and of conferring with my 

 brethren of the Royal Society and of the National Institute. 

 The climate of Provence is singularly adapted to such studies. 

 I find, by my journal of 1848, that during forty-six days 

 which I spent in those experiments, from 8 A.M. to 3 P.M., I 

 scarcely ever was interrupted by a cloud, although it was 

 November and December.* I have since had the great 

 benefit of a most excellent set of instruments made by 

 M. SOLEIL of Paris, whose great ingenuity and profound 

 knowledge of optical subjects can only be exceeded by his 

 admirable workmanship. I ought however to observe, that 

 although his heliostate is of great convenience in some expe- 

 riments, it yet is subject (as all heliostates must be) to the 

 imperfection of losing light by reflexion, and consequently I 



* Of seventy-eight days of winter in 1849, 1 had here only five of cloudy 

 weather. Of sixty-one days of summer at Brougham, I had but three or 

 four of clear weather ; one of these fortunately happened whilst Sir D. 

 Brewster was with me, and he saw the more important experiments. 



