12 CRYSTALLOGRAPHY. 



The wheel, n, is connected with an axis which passes through the 

 main axis (which is hollow for the purpose), and moves merely the 

 parts to which the crystal is attached, in order to assist in its adjust- 

 ment. The contrivances for the adjustment of the crjstal are at #, &, 

 c, d, k. The screws, c, d, are for the adjustment of the crystal, and tha 

 slides, a, b, serve to centre it. 



To use the instrument, it may be put on a stand or small table, with 

 its base accurately horizontal, and the table placed in front of a win- 

 dow, six to twelve feet off, with the plane of its circle at right angle* 

 to the window. A dark line must then be drawn below the window, 

 near the floor, parallel to the bars of the window, and about as far 

 from the eye as from the window-bar. 



The crystal is attached to the movable plate k by means 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, or by means of the 

 adjacent screws and slides. 



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 correctly adjusted, the selected bar 

 will appear horizontal, and on turning the wheel ?i, 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. The eye for both observations should be held in 

 precisely the same position. If there is not a perfect coincidence, the 

 adjustment must be altered until this coincidence is obtained. Con- 

 tinue then the revolution of the wheel n. 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 com- 

 plete ; if not, alterations must be made, and the first face again tried. 

 In an instrument like the one figured, the circle is usually graduated 

 to twenty or thirty minutes, and, by means of the vernier, minutes and 

 half minutes are measured. After adjustment, 180° on the arc must 

 be brought opposite 0, on the vernier, v. The coincidence of the bar 

 and dark line is then to be obtained, by turning the wheel n. When 

 obtained, the wheel m should be turned until the same coincidence is 

 observed, by means of the next face of the crystal. If a line on the 

 graduated circle now correspords 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 wtiich 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", the required angle is 125° 18' ; if this latter line marks 125° 20 , 

 the required angle is 125° 38'. 



In the better instruments other improved methods of arrangement 

 are employed ; and in the best, often called Mitscherlich's goniometei, 

 because first devised by him, there are two telescopes, one for passing 

 a ray of light upon the adjusted crystal, having crossed hair lines in its 

 focus, and the other for viewing it, also with a hair cross. With such 

 an arrangement, the window-bar and dark line are unnecessary, the 

 hair crosses serving to fix the position of the crystal, and the telescopa 

 that of the eye. If the crystal is perfect in its planes, and the adjust- 



