ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



461 



carbon ; and also by Dr. Zenker, who replaced the lic[uid in M. Jamin's 

 construction by wedges of flint glass. 



The following tabular view of different forms of polarizing prisms 

 is taken from the conclusion of Dr. Feussner's paper : — 



I. The Old Polabizing Peisms. 



1. Nicol's prism 



2. Shortened Nicol prism. 



a. Cemented with Canada balsam 

 h. „ „ copaiba „ 



3. Nicol with perpendicular ends. 



a. With Canada balsam 



b. With cement of index of refrac-\ 



tion of 1-525 / 



4. Foucalt's prism 



5. Hartnack's prism. 



a. Original form 



6. With largest field 



c. With field of 30° 



d. With field of 20° .. .. . 



6. Glan's prism 



II. Feussner's Polarizing Prism. 



1. With calc-spar : largest field 



2. „ „ field of 30° 



3. „ „ field of 20° 



4. With nitrate of soda ; largest field 



5. „ „ „ field of 30° 



6. „ „ „ field of 20° 



Field. 



29 



13 

 24 



20 



27 



35 

 41- 

 30 



20 



7' 



44 

 30 

 20 

 54 

 30 

 20 



Inclination 



of section 



in regard 



to long 



axis. 



22 



25 

 25 



15 

 15 

 40 



15 

 13 

 17 



20 

 50 



13-2 



17-4 



20-3 



16-7 



24 



27 



Ratio of 



length to 



clear 



width. 



3-28 



2-83 

 2-83 



3-73 

 3-73 

 1-528 



3-51 

 4-04 

 3-19 

 2-70 

 0-831 



4-26 

 3-19 

 2-70 

 3-53 

 2-25 

 1-96 



Fig. 



6Q 



67 

 67 



68 

 68 

 69 



70 a & 

 70 a a 

 70 a c 

 70 ad 

 71 



70 a a 

 70 a c 

 70 ad 

 72 a a 

 72 abSc 73 

 72 a c 



As an analysing 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^ in. — could be obtained with all the con- 

 venience of a Nicol would be a real advance, for spar of sufficient 

 size and purity for such a purpose has become so scarce, and there- 

 fore so valuable, that large prisms are difficult to procure at all. 

 So far as an analyser is concerned, the experience of Mr. P. R. 

 Sleeman 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 * are in this direction ; but crystals of 



* Phil. Mag., 1852, p. 161, and 1853, p. 346. 



