458 



SUMMARY OF CURRENT RESEARCHES RELATING TO 



Fig. 67. 



Fig. 68. 



2. The Shortened Nicol Prism (fig. 67).-^This arrangement of the 

 Nicol prism is constructed by Steeg and Eeuter of Homburg v. d. H. 

 For the sake of facility of manufacture, the end 

 surfaces are cleavage planes, and the oblique cut, 

 instead of being perpendicular, makes with these an 

 angle of about 84°. By this alteration the prism 

 becomes shorter, and is now only 2 "83 times its 

 breadth ; but if Canada balsam is still used as the 

 cement, the field will occupy a very unsymmetrical 

 position in regard to the long axis. If balsam of 

 copaiba is made use of, the index of refraction of which 

 is 1*50, a symmetrical field of about 24° will be ob- 

 tained. A prism of this kind has also been designed 

 by B. Hasert, of Eisenach,* but its performance appears 

 to be inferior to the above. 



3. The Nicol Prism with Perpendicular Ends 

 (fig. 68). — The terminal surfaces in this prism are 

 perpendicular to the long axis, and the sectional cut 

 makes with them an angle of about 76°. The length 

 of the prism is 3 • 76 times its breadth, and if the 

 cement has an index of refraction of 1 • 626, the field 

 is symmetrically disposed, and includes an angle 

 of 27°. Prisms of this kind have been manufactured 

 by Steeg, C. D. Ahrens, and others. 



4. The Foucault Prism f (fig. 69). — This construc- 

 tion differs from all those hitherto mentioned, in that 

 a film of air is employed between the two cut 

 surfaces as the totally reflecting medium instead of 

 a layer of cement. The two halves of the prism are 

 kept in position, without touching each other, by 

 means of the mounting. The length of the prism is in 

 this way much reduced, and amounts to only 1'628 

 times its breadth. The end surfaces are cleavage 

 planes, and the sectional cut makes with them an 

 angle of 69°. The field, however, includes not more 

 than about 8°, so that this prism can be used only 

 in the case of nearly parallel rays ; and in addition 

 to this the pictures which may be seen through it 

 are to some extent veiled and indistinct owing to 

 repeated internal reflection. 



5. The Hartnach Prism f (fig. 70). — This form of 

 prism was devised in 1866 by Hartnack and Praz- 

 mowski, and was described, vol. iii. (1883) p. 428. 

 It is considered by Dr. Feussner to be the most 

 perfect prism capable of being prepared from calc- 



spar. The ends of the prism are perpendicular to its length ; the 



Fig. 69. 



* Pogg. Ann., cxiii. p. 189. 



+ Comptes Eendus, xlv. (1857) p. 238. 



t Ann. Chem. et Physique, vii. (1866) p. 181. 



