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:
| Field. | Inclination of section in regard to long axis. | Ratio of length to clear 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 balsam. | 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 | 5ab | |
| b. With largest field. | 41.9 | 13.9 | 4.04 | 5aa | |
| c. With field of 30°. | 30 | 17.4 | 3.19 | 5ac | |
| d. With field of 20°. | 20 | 20.3 | 2.70 | 5ad | |
| 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 | 5aa |
| 2. " | field of 30°. | 30 | 17.4 | 3.19 | 5ac |
| 3. " | field of 20°. | 20 | 20.3 | 2.70 | 5ad |
| 4. With nitrate of soda: | largest field. | 54 | 16.7 | 3.53 | 7aa |
| 5. " | field of 30°. | 30 | 24 | 2.25 | 7ab |
| 6. " | field of 20°. | 20 | 27 | 1.96 | 7ac |
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.
PHILIP R. SLEEMAN.
ZIRCON.
By F. STOLBA.
Finely ground zircon is quickly rendered soluble if fused with a mixture of potassium borofluoride and potassium carbonate. The author takes two parts of the former to three of the latter, and prepares an intimate, finely divided mixture, which is kept ready for use.