348 
William Henry Hardinge, Esq., was elected a Member of the Com- 
mittee of Antiquities in the room of Charles Haliday, Esq., who had re- 
signed his place on that Committee. 
ProFEssoR JELLETT read to the meeting the following paper, describ- 
ing— 
A NEW ANALYZING PRISM. 
In determining the plane of polarization of a ray by means of the ordi- 
nary Nicols’ prism, the observer is required to arrest the rotation of the 
prism at the point at which the intensity of the transmitted light is a 
minimum. But it is difficult to do this with very great accuracy, inas- 
much as the observer is obliged to compare a shade of colour, uot with 
any other shade which is before his eyes at the same instant, but with 
his recollection of a shade observed at the previous instant. To insure 
any tolerable degree of accuracy, the observation must be made very ra- 
pidly, so that the eye may receive the new impression while the former 
one is still quite fresh inthe memory. The difficulty of doing this with 
accuracy in any case is obvious, but it is most felt in experimenting on _ 
light reflected or transmitted by fluids. For here it is impossible to 
touch the instrument without producing a tremulous motion in the fluid, 
and, therefore, in the image reflected or transmitted; and this motion, 
while it lasts, renders accurate observation very difficult. But if the ro- 
tation of the analyzing prism be stopped for a sufficient length of time 
to allow this motion to cease, the recollection of the previously existing 
tint will no longer be so fresh as to allow the comparison to be made 
with any very great exactness. The difficulty will be increased, as is 
easily seen, when there is any amount of elliptic polarization in the 
light which is to be examined. 
The remedy for this difficulty is to be sought in the construction of 
an analyzer in which the tints compared shall be s¢multaneous, not con- 
secutive, and the double quartz plate of M. Arago is an attempt to re- 
alize this conception. It is not necessary to examine particularly this 
instrument, which is wholly different in principle and 
(the author believes) very inferior in accuracy to that 
which is now laid before the Academy, and which is 
constructed as follows :— 
A rhombic prism of Iceland spar, whose longitu- 
dinal edges should have a length of about two inches, 
or a little more, is cut by two planes perpendicular to 
those edges, so as to form a right prism, as in Fig. 1. 
This prism is divided by a plane parallel to the edges, 
and making a small angle with the longer diagonal of 
the base; one of the two parts into which the prism 
is thus divided is then reversed, so as to place the base uppermost; the 
