1890-91.] 
Prof. Tait on Dr Bang’s Paper. 
33 7 
Not© on Dr Sang’s Paper. By Prof. Tait. 
(Read November 23, 1891.) 
At the very urgent request of the late Dr Sang, who regarded the 
above paper as one of his chief contributions to science, I brought 
before the Council of the Society the question of its publication. 
From the Minute-Book of the Ordinary Meetings, I find that it was 
read on the 20th February 1837, though it is not mentioned in the 
published Proceedings of that date. On 21st July 1891 the Council 
finally resolved that tbe paper should be printed in the Proceedings 
“ if otherwise found desirable.” The reasons in favour of printing 
it seem to outweigh those which may, readily enough, be raised 
against such a course. 
The subject is one with which, except of course in its elements, 
I have long ceased to be familiar. But, from the imperfect 
examination which I have found leisure to make, I have come 
to the following conclusions. 
The paper contains a very important suggestion which (one would 
have thought) should have been forthwith published, whatever 
judgment might be passed on the rest of the work: — viz., the pro- 
posal to construct the polariser of two glass prisms, separated by a 
thin layer, only, of Iceland spar. In view of the scarcity of this 
precious substance, such a suggestion was obviously of great value. 
I am not sufficiently acquainted with the early history of the Nicol 
prism to be able to pronounce on the question of Dr Sang’s claim 
to priority in the explanation of its action : — but he told me that he 
believed himself to have been the first to demonstrate that the separa- 
tion effected was due to the total reflection of the ordinary ray. 
And it is quite certain that, long subsequent to 1837, various very 
singular attempts at explanation have been given in print. The 
inventor, himself, seems to have thought that the effect of his 
instrument was merely to “increase the divergency” of the two rays. 
The numerical error which Dr Sang has pointed out in Malus’ 
work seems to have been a slip of the pen only, as the minutes and 
seconds of the angle in question are correctly given. He supplies no 
reference to the passage, but I find it in the list of calculated angles 
at p. 125 of the Theorie de la Double Refraction. It cannot be a 
mere misprint, because the supplement is given along with the angle, 
