J. M. Blake on Measuring the angles of Crystals. 309 
of symbols having reference to any system of crystallization. 
By thus making the measurement of a crystal, simply a me- 
chanical operation, this description will be sufficient to determine 
again the same species, so far as it can be done by angular 
ements of the planes. 
e 
distance of each plane ina zone, from one of them fixed upon 
their points of intersection. . ; 
_ Since a zone is a number of planes making parallel intersec- 
ions with each other, if we adjust any two of the planes for 
measurement, we have at the same time adjusted all, an 
rely go on and record the reading of the instrument for each 
Plane, 
_We thus preserve the relative position of the planes, which 
ives the measurements a value which cannot be attaine 
the ordinary method of separate measurements, for the location 
of meio we have to depend upon our knowledge of certain 
Symbols, 
It can be seen that there is thus a saving of labor, and of 
Space in recording, if we consider that the object in view is to 
8et a complete measurement of a crystal. 
iS measurement of zones may be carried to an extent 
Which convenience ‘will determine for each species. Four or 
five zones will often include nearly all the planes on a crystal, 
Perhaps all that would be actually needed for the determination 
To show what can be done by this method, suppose in the 
case of sulphate of copper, (CuOSO,+5HO) each plane to 
have its opposite, there will be more than twenty-eight planes 
Upon a complete crystal. ; 
_ All of these planes can be brought into three zones intersect- 
a in a common plane (3, of Rammelsberg). ; 
t will be necessary to add a fourth zone, crossing the others, 
to bind them together and make the description complete. Th 
Species, sulphate of copper, is selected, because it belongs to the 
triclinic system of crystallization, and therefore it might be con- 
Sidered more difficult of description than the average of species, 
and the remeasurement of a crystal for comparison, an op- 
