This prism differs very considerably from the preceding forms, and
consists of a thin plate of a doubly refracting crystal cemented
between two wedge-shaped pieces of glass, the terminal faces of which
are normal to the length. The external form of the prism may thus be
similar to the Hartnack, the calc-spar being replaced by glass. The
indices of refraction of the glass and of the cementing medium should
correspond with the greater index of refraction of the crystal, and
the directions of greatest and least elasticity in the latter must
stand in a plane perpendicular to the direction of the section. One of
the advantages claimed for the new prism is that, it dispenses with
the large and valuable pieces of spar hitherto found necessary; a
further advantage being that other crystalline substances may be used
in this prism instead of calc-spar. The latter advantage, however,
occurs only when the difference between the indices of refraction for
the ordinary and extraordinary rays in the particular crystal made use
of is greater than in calc-spar. When this is the case, the field
becomes enlarged, and the length of the prism is reduced.
[Illustration: Fig. 7.]
The substance which Dr. Feussner has employed as being most suitable
for the separating crystal plate is nitrate of soda (_natronsalpeter_),
in which the above-mentioned values are [omega] = 1.587 and [eta] =
1.336. It crystallizes in similar form to calcite, and in both cases
thin plates obtained by cleavage may be used.
As the cementing substance for the nitrate of soda, a mixture of gum
dammar with monobromonaphthalene was used, which afforded an index of
refraction of 1.58. In the case of thin plates of calcite, a solid
cementing substance of sufficiently high refractive power was not
available, and a fluid medium was therefore employed. For this purpose
the whole prism was inclosed in a short glass tube with airtight ends,
which was filled with monobromonaphthalene. In an experimental prism a
mixture of balsam of tolu was made use of, giving a cement with an
index of refraction of 1.62, but the low refractive power resulted in
a very considerable reduction of the field. The extent and disposition
of the field may be varied by altering the inclination at which the
crystal lamina is inserted (Fig. 7), and thereby reducing the length
of the prism, as in the case of the Hartnack.
In order to obviate the effects of reflection from the internal side
surfaces if the prism, the wedge-shaped blocks of glass of which it is
built up may be made much broader than would otherwise be necessary;
the edges of this extra width are cut obliquely and suitably
blackened.
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 3½ 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.