ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 511 



MICROSCOPY. 



A. Instruments, Accessories, etc.* 

 (1) Stands. 



Microspectroscope in Mineralogy.! — E. T. Wherry thinks that the 

 possibilities of the microspectroscope in the identification of minerals 

 and iu the study of their composition have apparently not been generally 

 appreciated by mineralogists, and his paper comprises descriptions of the 

 spectra of a much larger number of minerals than have heretofore been 

 examined. The apparatus he found most satisfactory was a Crouch 

 binocular Microscope stand, fitted with a 37-mm. objective, an Abbe- 

 Zeiss Spektral-ocular in the right-hand tube, and in the other an 

 ordinary low-power eye-piece, marked on the lower lens at the point 

 where the image of a mineral grain falls when it is visible through the 

 spectroscope slit. The prism which diverts part of the light into the 

 left-hand tube is withdrawn after the mineral grain has been centred, 

 so as to permit as much light as possible to pass through the spectro- 

 scope. A binocular Microscope is not absolutely necessary, but frequent 

 re -adjustments of the scale and slit have to be made if the mineral is 

 observed by swinging out the upper part of the spectroscope and the 

 slit-holder. Light may be obtained from any source yielding a brilliant 

 white light, such as a Welsbaeh burner or a Nernst lamp, although sun- 

 light or daylight are objectionable on account of the Fraunhofer bands. 

 For the study of minerals in thin sections the light is reflected up 

 through the specimen, but in the majority of cases better results are 

 obtained by concentrating the light laterally on the specimen by a lens 

 or by a parabolic mirror and observing the brightest portion of its path. 

 Not only does the latter plan yield the better spectra, but it permits the 

 examination of crystals on the matrix, and gems in their settings. To 

 set the wave-length scale of the instrument accurately, a sodium flame is 

 used, scale-division 058 '9 being brought into coincidence with the 

 yellow (D) line. In addition, a small slip of didymium glass, which 

 can be readily inserted at the opening where light for the comparison 

 spectrum enters, is very convenient, the interval between the strong- 

 absorption bands of neodymium and praseodymium in the yellow being 

 set at about 058 (580 ftp). The scale of the instrument is graduated 

 in hundredths of mikrons, but, except at the extreme red end, tenths of 

 divisions can be readily estimated, and it is most convenient to state 

 measurements in three-figure wave-lengths. The light diffused by 

 mineral grains shows in most cases more intense absorption-bands than 

 that transmitted through them, yet it must penetrate considerably to be 



* This subdivision contains (1) Stands ; (2) Eye-pieces and Objectives ; (3) 

 Illuminating and other Apparatus; (4) Photomicrography; (5) Microscopical 

 Optics and Manipulation ; (6) Miscellaneous. 



Smithsonian Misc. Coll., lxv. No. 5 (publ. 2362). 



2 N 2 



