27(> Prof. W. Gr. Adams on Measuring Polariscopes. 



instrument (which is placed with its axis vertical) through the 

 first lens B, which is fixed on the tube in which the polarizer 

 C is placed. 



D, E, F, G, and H (fig. 2) are lenses through which the light 

 passes ; and K is a Nicol's prism (the analyzer). This part of 

 the instrument forms a complete table-polariscope of consider- 

 able range. The NicoPs prism C and the lenses are supported 

 each by means of two screws (shown in fig. 3), "which may be 

 moved upwards or downwards in two slots, and the lenses fixed 

 in their proper positions. Between the plano-convex lenses E 

 and F are the two central lenses L, M, two portions of a sphere, 

 between w r hich the crystal to be measured is placed. The crystal 

 should be immersed in oil. The setting of the lens L has a screw 

 on its surface which fits into a screw in the setting of the other 

 lens M, the arm supporting the lenses being a flat piece of 

 metal one sixth of an inch thick, which is placed between them 

 before they are screwed together. This arm, I, is supported 

 by a stout crosspiece T, in the form of the arc of a circle which 

 has its centre at the centre of curvature of the two lenses: the 

 arc subtends nearly a right angle at the centre. It passes 

 through and is supported by two guides P, Q, and has on its 

 outer surface a rack which works with a small toothed wheel 

 turned by the milled head N. 



On the upper face of the arm I is a train of toothed wheels, 

 the setting of the lens M being provided with teeth, so that, by 

 means of the milled head S on the outer wheel, the lenses and 

 crystal may be turned about the axis of the instrument. 



The guides P and Q are attached to the face of a vertical 

 circle which may be fastened by screws to a circular graduated 

 rim R ; this rim is supported on and turns upon another fixed 

 vertical circle, which is provided with verniers, so that the 

 angle through which the rotating circle is turned can be mea- 

 sured to one minute. The fixed circle is fastened to and car- 

 ried by the piece of tubing holding the two lenses E and F, 

 which is supported between the two pieces of tubing carrying 

 the upper and lower lenses. 



In this instrument the crystal can be turned about each of 

 three axes which are mutually at right angles to one another. 



I. By turning the vertical circle and tubing to which it is 

 attached, the crystal is turned about the vertical axis of the 

 instrument. 



II. By turning the train of wheelwork the crystal can also 

 be turned about an axis passing centrically through the cen- 

 tral lenses, and the plane of its optic axes may be brought into 

 the desired position for measurement. 



III. By turning the arc T by means of the milled head 



