ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 



433 



of the spar, wliicla assures, according to what we have above stated, 

 the absolute maximum of angular extent of the field with cementing 

 substances whose index very nearly approaches the extraordinary 

 index of the spar in the plane normal to the crystalluie axis. 



It must not be forgotten that the figure of the new prism is that 

 which is really adopted in our instruments. Although the length of 

 the longest side of the Nicol prism is only 3 ■ 7 (that of the smallest 

 being taken as unity), the acute angles of this prism give it a length 

 which is four times that of the small side. 



Fig. 81 represents the new prism A B C D, with its section A C in 

 a plane perpendicular to the axis of the crystal cut out of the parallele- 

 piped ahcd. This construction requires a piece of spar larger than 

 what is required in the Nicol prism. The same piece of spar, how- 

 ever, would only give a Nicol prism efg h of nearly the same thick- 

 ness as the new one but which would be more than a third longer than 

 the latter, and that with a field of a third less in extent. 



It is above all as an analyser that the new prism presents great 

 advantages over the old form. It may, for instance, be placed very 

 conveniently between the eye and the eye-piece of a Microscope with- 

 out reducing the field of view ; whilst the Nicol prism not only 

 narrows directly the field in a notable proportion but also hinders the 

 observer from approaching his eye sufficiently near to the point where 

 the rays cross, an indispensable condition for embracing the whole 

 field at one view." 



In Hartnack's analyser of recent construction (fig. 82) the 

 mounting is united to the eye-piece b c, and there is a graduated disk a, 

 in which the tube with the lenses and the analysing prism can be 



Fig. 82. 



Frazmowski (A B D) and Nicol (efgh) 

 prisms. A section of the crystal perpen- 

 dicular to the axis mm'. 



rotated. The pointer d, which rotates with the prism, indicates the 

 angular magnitude of the revolution which has beeu made. 



Leitz, and Seibert and Krafft have also constructed this apparatus of 

 Hartnack's, adding a vernier and crossed wires ; and Merz, Wasser- 



Ser. 2.— Vol. III. 2 F 



