[Illustration:  Fig. 8.]

Dr. Feussner remarks that a prism similar in some respects to his new arrangement was devised in 1869 by M. Jamin (Comptes Rendus, lxviii., 221), who used a thin plate of calc-spar inclosed in a cell filled with bisulphide of carbon; and also by Dr. Zenker, who replaced the liquid in M. Jamin’s construction by wedges of flint glass.

Among others, the carefully considered modifications of the 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: 

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|      |Inclina- |Ratio |
|      |tion of  | of   |
|      |section  |length|
|      |in regard| to   |
|      |to long  |clear |
|Field.|axis.    |width.|Fig.
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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:            |      |         |      |
"         largest field.  | 54   |   16.7  | 3.53 |7 a a
5.       "         field of 30 deg..   | 30   |   24    | 2.25 |7 a b
6.       "         field of 20 deg..   | 20   |   27    | 1.96 |7 a c
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As an analyzing prism of about 6 mm. clear width, and 13.5 mm. long, the new prism is stated by its