350 4 
zation is known, and turning the analyzing prism until the tints become 
equal. The beam whose plane is required is then introduced, and when 
the equality of tints has been re-established, the angle through which 
the prism has revolved, read off on a graduated circle, gives the incli- 
nation of the required plane to the known plane. This mode of deter- 
mining the zero, a process which for perfect accuracy ought to be re- 
peated with each new set of observations, possesses the advantage of 
eliminating the personal equation of the observer. In examining a 
beam of any considerable magnitude, there will be found in different 
persons a tendency to think one part of the image darker than another, 
even when there is no real difference. With different observers, and 
even in the same person at different times, the part of the image thus 
preferred may be different, and if the zero were determined once for all, 
this might occasion sensible error. But as, in the method here given, 
such a preference will equally affect the position of the zero, it can have 
no influence on the final result. 
In the first prism which the author procured to be constructed, the 
angle between the planes CP, CP’ was about 7°. With this prism the 
range of error in the determination of a plane of polarization was 7’, the 
light employed being the diffused light of the sky. Although this was a 
very much smaller range than the author had ever been able to attain 
with a Nicol’s prism, it seemed that a greater amount of accuracy might 
be obtained, and as the brightness of the image appeared to be too great, 
a prism was constructed in which this angle was but half of its former 
value. With this prism, and with the same kind of light, the author 
finds that he can determine the position of the plane of polarization to LV’. 
With direct solar light, and a prism in which the planes are still 
closer, a greater degree of accuracy may certainly be obtained; in fact, 
it can be shown that by diminishing this angle and increasing the 
brightness of the light, so as to preserve unchanged the intensity of the 
image, the sensibility of the prism will vary as cot oi being the angle 
in question. 
The author has given direction for the construction of an instrument 
in which this angle shall be one-half of that last mentioned, but as the 
polarimeter at his disposal is not made to measure an angle less than 1’, 
a greater degree of accuracy would not be of any practical advantage. 
The prisms here described were executed by Mr. J. Bryson, of Prince’s- 
street, Edinburgh, in a manner which left nothing to be desired. 
Professor Kinahan read.a paper, entitled, ‘‘ Descriptions of Crango- 
nide and Galatheide inhabiting the Irish seas, with Observations on 
the general homologies of the groups, and the ‘propriety of subdividing 
the Britannic species of the former.” . 
Str Witt1am Rowan Hamitron, LL. D., M.R. I. A., communicated 
the following portion of his paper— 
