MEASUREMENT OF CRYSTALS. 51 



firmly arranged, is to be placed in front of a window, distant 

 from the same from six to twelve feet, and with the axis of 

 the instrument parallel to it. Preparatory to operation, a 

 dark line must be drawn below the window near, the floor, 

 parallel to the bars of the window ; or, what is better, on a 

 slate or board placed before the observer on the table. 



The crystal is attached to the movable plate, q, by a piece 

 of wax, and so arranged that the edge of intersection of the 

 two planes forming the required angle, shall be in a line with 

 the axis of the instrument. This is done by varying its 

 situation on the plate, q, or the situation of the plate itself, or 

 by means of the adjacent joints and wheel, r, s, p, as will be 

 readily understood from the instrument. 



When apparently adjusted, the eye must be brought close 

 to the crystal, nearly in contact with it, and on looking into 

 a face, part of the window will be seen reflected, one bar of 

 which must be selected for the trial. If the crystal is cor- 

 rectly adjusted, the selected bar will appear horizontal, and 

 on turning the wheel, n, till this bar, as reflected, is observed 

 to approach the dark line below, seen in a direct view, it will 

 be found to be parallel to this dark line, and ultimately to 

 coincide with it. If there is not a perfect coincidence, the 

 adjustment must be altered until this coincidence is obtained. 

 Continue then the revolution of the wheel, w, till the same 

 bar is seen by reflection in the next face, and if here there 

 is also a coincidence of the reflected bar with the dark line 

 seen direct, the adjustment is complete ; if not, alterations 

 must be made, and the first face again tried. A few succes- 

 sive trials of the faces, will enable one to obtain a perfect 

 adjustment. 



The circle A B is usually graduated to half degrees, and 

 by means of the vernier, r, minutes are measured. After 

 adjustment, 180° on the arc must be brought opposite 0, on 

 the vernier. The coincidence of the bar and dark line is 

 then to be obtained, by turning the wheel, n. When ob- 

 tained, the wheel, m, should be turned until the same coinci- 

 dence is observed, by means of the next face of the crystal. 

 If a line on the graduated circle now corresponds with on 

 the vernier, the angle is immediately determined by the 

 number of degrees opposite this line. If no line corresponds 

 with 0, we must observe which line on the vernier coincides 

 with one on the circle. If it is the 18th on the vernier, and 

 the line on the circle next below on the vernier marks 125° 



