13 



nicol as polarizer, and a second nicol as analyzer, which carries a 

 graduated circle and contains the essential part of the instrument. 

 This is the plate fixed below the analyzer, consisting of two 

 slices of calcite cut nearly, but not quite, perpendicularly to the 

 axis, and placed one over the other, so that their optical axes lie 

 in the same plane but yet do not coincide. These produce, when 

 viewed alone, an interference figure of peculiar form, with a 

 straight arm of a cross through the centre. When, however, a 

 crystal plate is laid on the stage, unless its plane of vibration 

 coincides with those of one or other of the nicols, this figure is 

 altered by the dislocation of the central part of the arm of the 

 cross. The amount of rotation necessary to restore the figure, 

 and so make the planes of vibration coincide with those of the 

 nicols can be measured on the graduated stage. 



45. Kohlrausch's total reflectometer, with, improve- 

 ments by Klein. 



E. 145. 1886. Made ly W. Apel, Gottingen. 



This instrument is devised for the purpose of determining the 

 refractive indices of crystalline minerals and other substances by 

 comparing them with a fluid of known higher refractive index. 

 It consists of a triangular foot with the sides hollowed out so 

 that a source of light may be brought quite close to the centre 

 and on one side an upright beam, to the summit of which is 

 fixed, by a horizontal side support, a circle graduated in half 

 degrees and movable about a central axis, and with a fine 

 adjustment screw at the side. The readings are made by two 

 magnifiers and verniers. To the under side of the circle opposite 

 the vertical beam is fixed a horizontal telescope at the end of a 

 vertical arm. A glass jar, one side of which has a piece of plate 

 glass fixed vertically against it, is filled when in use by the dense 

 liquid employed, either carbon bisulphide, or bromnaphthaline, and 

 is supported in slots on the under side of the horizontal ring, so 

 that when placed in position the plate glass is opposite the tele- 

 scope. Through the centre of the axis of the horizontal circle runs 

 a vertical bar, which is attached below by a pair of ball and socket 

 joints to the crystal carrier, and is movable up and down in 

 its socket until fixed by a screw near the top. This crystal 

 carrier is a circular frame, to the surface of which the crystal 

 may be attached, and which is rotated by means of an endless 

 screw working on its circumference and attached to a rod with 

 a Hooke's joint passing through an aperture in the horizontal 

 circle. The object of these arrangements is to place the surface 

 of the crystal to be examined in the same plane as the axis of 

 the circle, to test which a mirror is attached to the outside 

 stem of the instrument, to which it must be placed optically 

 parallel. From the centre of the base of the instrument stands 

 up a paper holder in the shape of a double arc, and which may 

 be turned about its axis. The source of light is placed behind 



U 69149. 



