The table, or some easily recognizable facet, is selected as the facet at which light enters the stone. The telescope is first placed in the position in which it is directly opposite the collimator (_T_{0}_ in Fig. 23), and clamped. The scale is turned until it reads exactly zero, 0° or 360°, and clamped. The telescope is released and revolved in the direction of increasing readings of the scale to the position of minimum deviation, _T_. The reading of the scale gives at once the angle of minimum deviation, _D_. The holder carrying the stone is now clamped to the scale, and the telescope is turned to the position, _T_{1}_,in which the image given by reflection from the table facet is in the centre of the field of view; the reading of the scale is taken. The telescope is clamped, and the scale is released and rotated until it reads the angle already found for _D_. If no mistake has been made, the reflected image from the second facet is now in the field of view. It will probably not be quite central, as theoretically it should be, because the stone may not have been originally quite in the position of minimum deviation, a comparatively large rotation of the stone producing no apparent change in the position of the refracted image at minimum deviation, and further, because, as has already been stated, the method is not strictly true for biaxial stones. The difference in readings, however, should not exceed 2°. The reading, _S_, of the scale is now taken, and it together with 180° subtracted from the reading for the first facet, and the value of _A_, the interior angle between the two facets, obtained. Let us take an example. Reading _T_ (= _D_) 40° 41´ Reading T_{1} 261° 35´ less 180° 180 0 ——————— 81 35 Reading _S_ 41 30 ——————— ½_D_ 20 20½ _A_ 40 5 ½_A_ 20 2½ ½_A_ 20 2½ ——————— ½(_A_ + _D_) 40 23 Log sin 40° 23´ 9.81151 Log sin 20 2½ 9.53492 ——————— Log _n_ 0.27659 _n_ = 1.8906. The readings _S_ and _T_ are very nearly the same, and therefore we may be sure that no mistake has been made in the selection of the facets.