ZOOLOGY AND BOTANY, MICROSCOPY. ETC. 



633 



micrometer arrangement (M, 

 tig. 156). The advantage of 

 this determination method, in 

 contrast with the direct deter- 

 mination of the refractive in- 

 dex, consists in a considerable 

 simplification of the measure- 

 ment and in, under otherwise 

 similar conditions, great in- 

 crease of accuracy. The limits 

 of attainable accuracy for the 

 refractive index affect the 

 fourth decimal place to the ex- 

 tent of one unit, and for the 

 dispersion (difference of the 

 refractive indices) one or two 

 units in the fifth decimal place. 



The second telescope shown 

 in the figure is only occasion- 

 ally essential. 



Fig. 159 is a simpler form 

 of the same instrument ad- 

 apted for students' use in 

 physical laboratories. 



Zeiss' High. Temperature 

 Spectrometer. — Figs. 161, 

 162, show this instrument, 

 which is specially adapted for 

 the examination of the influ- 

 ence of temperature on the 

 refraction of solid bodies 

 (glasses, &c). It is appli- 

 cable to temperatures of 400° 

 C. and upwards. 



The tube A (fig. 162) 

 fastened on the rotation axis 

 of the spectrometer, carries a 

 small table T fitted with ad- 

 justable screws, on which is a 

 glass tube G with a lid, and 

 upon it the prism to be ex- 

 amined. The coarse rotation 

 is by hand movement, and the 

 fine by micrometer screw. The 

 warming of the object is at- 

 tained by heating a kind of 

 copper jacket, weighing about 

 50 kilos., with vertical cylin- 

 drical perforations ; within this 

 jacket the prism-carrier freely 

 rotates. The flame-gases from 

 the Bunsen burners pass up 



Oct. 17 th, 1900 



%%\% 



\ \ 



Fig. 161. 



•1 U 



