PHILOSOPHICAL TRANSACTIONS. 
XII. T he Bakerian Lecture. — On the Theoretical Explanation of an apparent new 
Polarity in Light. By G. B. Airy, Esq. M.A. F.R.S., Astronomer Royal. 
Received June 14, — Read June 18, 1840. 
IN the Report of the Seventh Meeting of the British Association and in that of 
the Eighth Meeting'!', David Brewster has given a short account of experiments 
which have led him to infer the existence of a polarity, in the rays of homogeneous 
light, having regard only to the sequence of colours in the spectrum. I am aware 
that Sir David Brewster has announced his intention of publishing a more elaborate 
paper upon these experiments. But as the leading facts of the experiments have now 
been communicated to the public nearly two years, and by Sir David Brewster 
himself, and as the accounts have been sufficiently explanatory to enable any other 
person to repeat the experiments, and as various persons have in consequence re- 
peated them, there cannot, I conceive, be the most trifling impropriety in making the 
whole subject a matter of discussion, experimental and theoretical. 
As the experimental facts observed by myself differ from those of Sir David Brew- 
ster in a degree which, as regards the experiment, is trifling (and might easily pass 
unobserved), but which, as regards the theory, is important ; I think it necessary to 
give the following brief history. My first repetition of the experiment was made in 
October, 1839, in consequence of a friend requesting my opinion upon a proposed 
theoretical explanation. To that friend I immediately communicated (by letter) the 
experimental results at which I had arrived, with a request that he would verify them ; 
and he did accordingly verify them, as far as the apparatus in his hands allowed. 
These results suggested to my mind the general train of the theoretical explanation ; 
and the numerical calculations necessary for following it into details were immedi- 
ately placed in the hands of a computer. The incessant occupations incidental to my 
office prevented me from even looking at the calculated numbers till the month of 
May, 1840, when the remarks of another friend recalled my attention to the subject. 
* Transactions of the Sections, p. 12. f Transactions of the Sections, p. 13. 
MDCCCXL. 2 G 
