Fig. 23.—Course of Observations in the Method of Minimum Deviation.
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 (T0 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, T1,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 T1 | 261° | 35´ |
|
|
| less 180° | 180 | 0 |
|
|
|
| ——————— | |
|
|
|
| 81 | 35 |
|
|
| Reading S | 41 | 30 |
|
|
|
| ——————— | |
½D | 20 | 20½ | A | 40 | 5 |
½A | 20 | 2½ | ½A | 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.
In place of logarithm-tables we may make use of the diagram on [Plate II]. The radial lines correspond to the angles of minimum deviation and the skew lines to the prism angles, and the distance along the radial lines gives the refractive index. We run our eye along the line for the observed angle of minimum deviation and note where it meets the curve for the observed prism angle; the refractive index corresponding to the point of intersection is at once read off.
This method has several obvious disadvantages: it requires the use of an expensive and elaborate instrument, an observation takes considerable time, and the values of the principal refractive indices cannot in general be immediately determined.
[Table III] at the end of the book gives the refractive indices of the gem-stones.
PLATE II
