990 The American Naturadist. [November, 
General Notes. 
MINERALOGY. 
An Instrument for Preparing Accurately Oriented Sec- 
tions and Prisms from Crystals.—Mention has been made in 
these notes of the valuable instruments which Tutton has designed in 
connection with his recent studies in the field of chemical crystallo- 
graphy. _One of them’ is an instrument of precision for preparing 
prisms or sections of the delicate crystals of artificially prepared com- 
pounds. The methods now in use for making these preparations re- 
quire a prodigious amount of labor while securing only a rough 
approximation to the desired orientation. Of his new instrument Tut- 
ton says: 
“It is possible by the use of the instrument to grind and polish a 
truly plane surface in any direction in a crystal so as to be true to that, 
direction to within ten minutes of are, an amount of possible error 
which would exercise no measurable influence upon the values of the 
optical constants. Moreover, this result may be achieved in a small 
fraction of the time hitherto required, and with only the very slightest 
risk of fracturing the crystal. An arrangement is also provided by 
which a second surface may be ground parallel with a like degree of 
accuracy to the first.” 
This somewhat elaborate piece of apparatus is constructed like an 
inverted goniometer with horizontal circle, being provided with grad- 
uated disc, the usual centering and adjusting device, telescope, colli- 
mator and lamp. A revolving table mounted in an excentric position 
under the crystal and driven by a turning table, carries a ground glass 
plate for grinding and a finer one for polishing. The pressure of the 
crystal on the glass is delicately regulated by means of counterpoised 
levers which support any desired portion of the weight of the instru- 
ment’s axis, the remaining portion bearing directly on the crystal. 
A larger, stronger, and somewhat modified form of this apparatus’ 
has been designed for carrying out the same operations on the hard 
natural crystals. This form is provided with a cutting apparatus, 
which, when not in use, is rotated out of the way so as not to interfere 
t Edited by Dr. Wm. H. Hobbs, University of Wisconsin, Madison, Wi. 
* Philosophical Transactions, Vol. 185, (1894), A, pp. 887-912. 
*Tutton, Proc. Roy. Soc., Vol. 57, pp. 324-330. 
