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AN INSTRUMENT FOR GRINDING SECTION-PLATES, ETC. 
indices of biaxial crystals may be determined, two in duplicate and one four times 
repeated upon the same crystal. 
The faces of the prisms need not be covered with thin glass plates, cemented by a 
solution of balsam in benzene, if a little time and trouble is taken to fully utilise the 
polishing disc. Moreover, prisms may be prepared by this method even from very 
small crystals, such as one could never hope to fit satisfactorily with cover glasses, 
and, if carefully polished, the brightness of the refracted images of the Websky slit 
of i-he spectrometer will be ample to enable accurate determinations of refractive index 
to be made. The images reflected by the polished surfaces furnished by the 
instrument are invariably well-defined and single, enabling excellent measurements of 
the angle of the prism, as well as of the angles of minimum deviation of the refracted 
rays, to be made. 
It may be remarked, in conclusion, that the instrument in the form described is 
somewhat too delicate to be employed for grinding sections of naturally-occurring 
crystals harder than glass, by substituting a small lapidary’s wheel for the ground- 
glass grinding disc. The author expects shortly to be able to describe an instrument, 
now in course of construction, specially adapted for preparing sections and prisms of 
mineral crystals. 
The author desires to express his thanks to the Research Fund Committee of the 
Chemical Society for the grant to defray the cost of the instrument. It has been 
made by Messrs. Troughton and Simms, to whom the author is very considerably 
indebted for assistance in devising it, and for the care bestowed on its construction. 
