04 TERMINOLOGY. 



the arms are applied to a crystal, they should be previously brought 

 sufficiently near together to form a more acute angle than that about 

 to be measured. The arms being then gently pressed upon the 

 crystal, they will gradually separate until they come to fit the 

 planes so exactly, that when held between the eye and the win- 

 dow, no light can be perceived to flow between them. When the 

 crystals are well adapted to these measurements, and in the hands 

 of a person familiar with the operation, results may be obtained 

 from any number of successive trials, whose greatest difference will 

 not be above a quarter of a degree. 



On the whole, the common goniometer, may be said to supply us 

 only with approximations to the real value of the angles of crystals, 

 in consequence of the frequent irregularities upon their planes, or 

 the minuteness of their planes, as well as the difficulty of applying 

 the arms in a direction perpendicular to the intersection of the planes 

 to be measured. Still, it is an instrument indispensable to the Min- 

 eralogist ; and is in general every way adequate, in point of the accu- 

 racy of its results, to be employed in the determination of minerals. 



2. Reflective Goniometer. 



Fig. 135. 



This instrument, which is of invaluable utility to the Mineralo- 

 gist, was an invention of the late Dr. Wollaston. It is represented 

 in Fig. 135. It consists of an entire circle, divided into degree* 



