1883.] 



MICROSCOPICAL JOURNAL. 



171 



cvanides, more particularly those of 

 magnesium and yttrium. 



Another peculiarity of some crystals 

 was in 1837 designated by Fox Tal- 

 bot as analytic, that is, they have the 

 power of analyzing polarized light 

 like a tourmaline, and when examined 

 on the microscope stage, do not need 

 the analyzer above the objective. 

 This power is possessed by the platino- 

 cyanides, boracic acid, murexide, hip- 

 puric acid, nitrate of potash, iodo-sul- 

 phate of quinine, and many others. 

 .The scientific cause of these powers 



of dichroism, analysis, and fluores- 

 cence, as well as polarization I pass 

 entirely by, leaving them to be dealt 

 with by others much more fitted for 

 the work. And in concluding, I would 

 ask any who are tempted to take up this 

 subject, either in a scientific or dille- 

 tantc manner, to jot down for their 

 own or other's benefit anything which 

 arises from their experiment, as it is 

 from such notes as those, made at my 

 work-table, that I have ventured to 

 prepare this paper, 



Fig. 30. — Illuminating Apparatl's for Monochromatic Light 



Illiimiiiatiiig Apparatus for 

 Mouocliroiiiatic Light. 



The use of monochromatic light is 

 frequently desirable in microscopic 

 work, especially blue light. The 

 usual method of obtaining colored 

 light is to pass sunlight through col- 

 ored glass, or through a colored solu- 

 tion, such as the ammonio-sulphate of 

 copper for blue light. Mr. Zeiss has 

 constructed an apparatus (after Hart- 

 nack ) for this purpose which is illus- 

 trated in fig. 30. By means of two 

 prisms P.' P'-., of strong dispersive 

 power a spectrum of considerable 

 length is projected upon the object 



from beneath, so that with high powers 

 the entire field is illuminated with a 

 near approach to monochromatic light. 

 The light enters the instrument 

 through the slit Sp, which is adjusta- 

 ble in w-idth by the screw ■5"-', and 

 passes through the prisms and the lens 

 at O forming a spectrum at Spk where 

 the object on the stage is supposed to 

 be situated. By moving the slit by 

 the screw ^', the spectrum is caused 

 to pass over the object, the different' 

 colors following in succession. The 

 instrument may be used for low-powers 

 with ordinary daylight, but for high- 

 powers sunlight must be employed. 



