890 
MR. A. E. TUTTON ON AN INSTRUMENT FOR GRINDING SECTION- 
tlie refractive indices, and the results ol^tained from them are no longer only approxi¬ 
mate, but precise. 
Construction o f the Instrument. 
A general view of the instrument and its principal accessories is given in fig. 1. 
It consists essentially of the following five parts :— 
1 . A rotatable horizontal divided circle and fixed vernier. 
2 . A suspended vertical axis, rotating with the circle and capable of vertical motion, 
carrying at the lower extremity the crystal and its means of adjustment. 
3. A rotatable grinding disc, whose surface is parallel to the circle and perpen¬ 
dicular to the suspended axis. 
4. A horizontal collimator and telescope, for goniometrically observing the crystal. 
5. An arrangement for wholly or partly I’elieving, or foi’ increasiug, the pressure 
with which the crystal bears upon the grinding disc during grinding. 
Upon a circular solid metal base are erected three brass columns, which support a 
