30 OF THE GONIOMETER. 



below the reflecting surface, as the object itself is 

 above it. 



If therefore the planes #, and b, fig. 57, be suc- 

 cessively brought into such positions, as will cause 

 the reflection of the line at ??z, from each plane, to 

 appear to coincide with another line at n, both planes 

 will be successively placed in the relative positions 

 of the corresponding planes in figs. 55 and 56. 



Fig. 58. 



To bring the planes of any crystal successively into 

 these relative positions by the assistance of the reflec- 

 tive goniometer, the following directions will be 

 found useful. 



The instrument, as shewn in the sketch fig. 58, 

 should be first placed on a pyramidal stand, and the 

 stand on a small steady table, placed about 6 to 10 or 

 12 feet from ajlat window. 



The graduated circular plate should stand perpen- 

 dicularly from the window, the pin x being horizontal^ 

 with the slit end nearest to the eye. * 



Place the crystal which is to be measured, on the 

 table, resting on one of the planes whose inclination is 



" This goniometer is sometimes drawn with the pin x in the direction of 

 its axis, in which position of the pin, the instrument may be regarded 

 as nearly useless. 



