Nicol
prism which have recently been devised by Prof. S.P. Thompson (_Phil.
Mag._, November, 1881, 349, and _Jour. R. Micros. Soc._, August, 1883,
575), and by Mr. R.T. Glazebrook (_Phil. Mag._, May, 1883, 352), do
not appear to have been known to Dr. Feussner.

The following tabular view of different forms of polarizing prisms is
taken from the conclusion of Dr. Feussner's paper:

---------------------------------------+------+---------+------+------
| |Inclina- |Ratio |
| |tion of | of |
| |section |length|
| |in regard| to |
| |to long |clear |
|Field.|axis. |width.|Fig.
---------------------------------------+------+---------+------+------
I. THE OLD POLARISING PRISMS. | deg. | deg. | |
1. Nicol's prism. | 29 | 22 | 3.28 | 1
2. Shortened Nicol prism-- | | | |
a. Cemented with Canada balsam.| 13 | 25 | 2.83 | 2
b. Cemented with copaiba " | 24 | 25 | 2.83 | 2
3. Nicol with perpendicular ends-- | | | |
a. With Canada balsam. | 20 | 15 | 3.73 | 3
b. With cement of index of | | | |
refraction of 1.525. | 27 | 15 | 3.73 | 3
4. Foucault's prism. | 8 | 40 | 1.528| 4
5. Hartnack's prism-- | | | |
a. Original form. | 35 | 15.9 | 3.51 |5 _a b_
b. With largest field. | 41.9 | 13.9 | 4.04 |5 _a a_
c. With field of 30 deg.. | 30 | 17.4 | 3.19 |5 _a c_
d. With field of 20 deg.. | 20 | 20.3 | 2.70 |5 _a d_
6. Glan's prism. | 7.9 | 50.3 | 0.831| 6
| | | |
II. THE NEW POLARISING PRISM. | | | |
1. With calc-spar: largest field. | 44 | 13.2 | 4.26 |5 _a a_
2. " field of 30 deg.. | 30 | 17.4 | 3.19 |5 _a c_
3. " field of 20 deg.. | 20 | 20.3 | 2.70 |5 _a d_
4. With nitrate of soda: