ZOOLOGY AND BOTANY, MIOROSCOPY^ ETC. 



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as suggested by Mr. J. Mayall, jun. ; and (4) an alteration in the form 

 of the tripod, so that when the instrument is vertical the crank-arm of 

 the mirror is allowed free lateral play. 



By this system of fine adjustment (which is patented) the binocular 

 prism is brought more than 1/4 inch nearer the posterior surface of 

 the objective-lenses than in any Microscope hitherto constructed by 

 Messrs. Swift. 



Sorby's Dichroiscope. — Dr. H. C. Sorby's dichroiscope (fig. 9), 

 as made by Messrs. Beck, consists of four parts : — (1) An A eye-piece, 

 between the two lenses of which slides a blackened brass plate with a 

 circular aperture and a slit. The width of the latter can be varied 

 by the small milled head 



acting on a spring. (2) A Fig. 9. 



double-image prism. (3) An 

 analyser; and (4) a direct- 

 image prism. As the whole 

 apparatus is somewhat heavy, 

 the tube of the eye-piece does 

 not terminate just below the 

 field lens, but is continued for 

 an inch further, so as to insure 

 a firm hold in the body-tube. 

 The field lens is attached to 

 a separate inner tube, which 

 slides (with a bayonet catch) 

 in the outer, so that it may be 

 readily removed for cleaning. 



The method of using the 

 apparatus is thus described by 

 Dr. Sorby (extracted from 

 letters from himself) : — 



" In the examination of 

 sections of granite and other 

 minerals with polarized light, 

 if the sliding plate be inserted 

 in the eye-piece and the 

 double-image prism placed 

 over the eye-piece under the 

 analyser, two images of the 

 hole or slit are seen, of different colours if the object on the stage is 

 dichroic, or if the colour is due to chemical change only one image 

 will be coloured. If now the direct-image prism of a spectroscope be 

 placed over the double-image prism and the analyser removed, the 

 two spectra can be seen side by side. 



The most useful application of the instrument is in stiidying 

 small crystals or sections of rocks. We can then tell whether 

 difference in colour is due to mere difference in position of the same 

 crystal or not ; for example, in many granites we see yellowish and 

 orange crystals mixed up with the black. By means of the dichroi- 

 scope we can see at once that these are the same mineral in different 



