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XIL Experiments and Observations upon the Properties of Light. 
By Lord Brougham, F.R.S., 
Member of the National Institute, and of the Royal Academy of Naples. 
Received November 6, 1849, — Read January 10, 1850. 
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 Pro- 
vence : they were further prosecuted in October. I had thus an opportunity of care- 
fully reconsidering the conclusions 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 iny 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 expe- 
riments, from 8 A.M. to 3 p.m., I scarcely ever was interrupted by a cloud, although 
it was November and December*. 1 have since had the great benefit of a most ex- 
cellent 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 work- 
manship. I ought however to observe, that although his heliostate is of great con- 
venience in some experiments, it yet is subject (as all heliostates must be) to the 
imperfection of losing light by reflexion, and consequently 1 have generally been 
obliged to encounter the inconvenience of the motion of the sun’s image, especially 
when I had to work with small pencils of light. This inconvenience is materially 
lessened by using horizontal prisms and plates. 
Although I have made mention of the apparatus of great delicacy which I em- 
ployed, it must be observed that this is only required for experiments of a kind to 
depend upon nice measurements. All the principles which I have to state as the 
result of my experiments in this paper, can be made with the most simple apparatus, 
and without any difficulty or expense, as will presently appear. 
It is perhaps unnecessary to make an apology for the form of definitions and pro- 
positions into which my statement is thrown. This is adopted for the purpose of 
making the narrative shorter and more distinct, and of subjecting my doctrines to a 
fuller scrutiny. I must further premise that I purposely avoid all arguments and 
* Of seventy-eight days of winter in 1849, I 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. 
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