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Page 38
The accompanying diagram (Fig. 8) represents a prism of cylindrical
external form constructed in this manner, the lower surface being that
of the incident light. In this the field amounts to 30�, and the
breadth is about double the length.
[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:
---------------------------------------+------+---------+------+------
| |Inclina- |Ratio |
| |tion of | of |
| |section |length|
| |in regard| to |
| |to long |clear |
|Field.|axis. |width.|Fig.
---------------------------------------+------+---------+------+------
I. THE OLD POLARISING PRISMS. | � | � | |
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�. | 30 | 17.4 | 3.19 |5 _a c_
d. With field of 20�. | 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�. | 30 | 17.4 | 3.19 |5 _a c_
3. " field of 20�. | 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�. | 30 | 24 | 2.25 |7 _a b_
6. " field of 20�. | 20 | 27 | 1.96 |7 _a c_
---------------------------------------+------+---------+------+------
As an analyzing prism of about 6 mm. clear width, and 13.5 mm. long,
the new prism is stated by its inventor to be of the most essential
service, and it would certainly appear that the arrangement is rather
better adapted for small prisms than for those of considerable size.
Any means by which a beam of polarized light of large diameter--say 3
to 3� inches--could be obtained with all the convenience of a Nicol
would be a real advance, for spar of sufficient size and purity for
such a purpose has become so scarce and therefore so valuable that
large prisms are difficult to procure at all. So far as an analyzer is
concerned, the experience of the writer of this notice would lead to
the opinion that improvements are to be looked for rather in the way
of the discovery of an artificial crystal which absorbs one of the
polarized rays than by further modifications depending upon total
reflection. The researches of Dr. Herapath on iodosulphate of quinine
(_Phil. Mag._, March, 1852, 161, and November, 1853, 346) are in this
direction; but crystals of the so-called herapathite require great
manipulative skill for their production. If these could be readily
obtained of sufficient size, they would be invaluable as analyzers.
This opinion is supported by the existence of an inconvenience which
attends every form of analyzing prism. It is frequently, and
especially in projecting apparatus, required to be placed at the focus
of a system of lenses, so that the rays may cross in the interior of
the prism. This is an unfavorable position for a prismatic analyzer,
and in the case of a powerful beam of light, such as that from the
electric arc, the crossing of the rays within the prism is not
unattended with danger to the cementing substance, and to the surfaces
in contact with it.
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