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CRYSTALLOGRAPHY.

AB is -die circle graduated to half degrees. By means of thevernier, v, minutes are measured. The wheel, m. is attached tothe main axis, and moves the graduated circle together with theadjusted crystal. The wheel, n, is connected with an axis whichpasses through the main axis, (which is hollow for the purpose,)and moves merely the parts to which the crystal is attached, in or-der to assist in its adjustment. The contrivances for the adjust-ment of the crystal, are at p, q, r. To use the instrument, it mustbe placed on a small stand or a table, and so elevated, as to allowthe observer to rest his elbows on the table. The whole, thus firmlyarranged, is to be placed in front of a window, distant from thesame from six to twelve feet, and with the axis of the instrumentparallel to it. Preparatory to operation, a dark line must be drawnbelow the window near the floor, parallel to the bars of the window.

The crystal is to be attached to the moveable plate, q, by a pieceof wax, and so arranged, that the edge of intersection of the twoplanes forming the required angle, shall be in a line with the axisof the instrument. This is effected by varying its situation on theplate, q , or the situation of the plate itself, or by means of the adja-cent joints and wheel, r, s, p.

When apparently adjusted, the eye must he brought close to thecrystal, nearly in contact with it, and on looking into a face, partof the window will be seen reflected, one bar of which must he se-lected for the experiment. If the crystal is correctly adjusted, theselected bar will appear horizontal, and on turning the wheel ji,till this bar as reflected, is observed to approach the dark line belowseen in a direct view, it will be found to be parallel to this darkline, and ultimately to coincide with it. If there is not a perfectcoincidence, the adjustment must be altered until this coincidenceis obtained. Continue then the revolution of the wheel, n, till thesame bar is seen by reflection in the next face, and if here there isalso a coincidence of the reflected bar with the dark line seen di-rect, the adjustment is complete ; if not, alterations must be made,and the first face again tried. A few successive trials of the twofaces, will enable one to obtain a perfect adjustment.

When adjusted, 180° on the arc must be brought opposite 0, onthe vernier. The coincidence of the bar and dark line is then tobe obtained, by turning the wheel n. As soon as obtained, thewheel, m, should be turned until the same coincidence is observed,by means of the next face of the crystal. If a line on the graduatedcircle now corresponds with 0 on the vernier, the angle is imme-diately determined by the number of degrees marked by this line.If no line corresponds with 0, we must observe which line on thevernier coincides with one on the circle. If it is the 18th on thevernier, and the line on the circle next below 0 on the verniermarks 125°, the required angle is 121° IS 7 ; if this line marks125° 30', the required angle is 125° 48'.

If the observer is near sighted, and incapable of seeing the dark