310 J. M. Blake on Measuring the angles of Crystals. 
eration requiring considerable patience, to say the least; but by 
the use of this method it becomes in either case a comparatively 
- operation. 
f, from the description, the angle be required between any 
two planes lying in one of the measured zones, it can be readily 
obtained by subtraction. But if the planes do not both lie in 
one of the measured zones the angle can be found by means of 
spherical trigonometry, without taking into consideration sym- 
bols or axes, 
The prismatic zone in elongated crystals would be the first to 
measure, because best developed. 
é can start with the zero of the instrument opposite the 
largest plane if we have no other guide. 
The other zones may be takem in the order in which the 
appear best developed. Some order is desirable to facilitate the 
use of the description for fature comparisons. 
In using the description for determining species, after measur- 
ing the most obvious zones on the crystal, we can plot them on 
paper and turn the protractor until the readings coincide with 
the description. ie A 
The plotting is necessary only when there is difficulty 
making the comparison, on account of the zero of the instru- 
ment not having been brought opposite the plane used in the 
description. . 
hen we have found a coincidence of readings for a zone We 
have determined all the planes in the zone; also the syinbols, if 
given in the description. : 
In this way, with a description in this form, we can speedily 
determine all of the given planes which occur upon the crystal 
before us; also if there are any new planes. ; 
Care should be taken not to confound right and left hemt 
hedrism, or similar non-superposdble forms, which is liable to be 
done if we do not note the relative position of at least three 
planes, not in one zone. In case a drawing is given, this W! 
not be necessary. 
e use of this method gives us a great advantage in detect- 
ing and measuring accurately very small planes and cleavages 
which, were it not for the fact that they come into the zones, 
could not be adjusted for measurement on account of the feeble 
reflection which they give. 
Note.—Direct sunlight is of great value in detecting and measuring 
small planes: the light may be tem means of smoked glass. 
0) 
length, and for most crystals one millimeter in di 
A 
: ameter, 
method for the rapid and easy adjustment of a crystal for measure- 
