FIG. 229. 



THE POLARISCOPE AS APPLIED TO SUGAR ANALYSIS. 



The original form of the nicol prism has been modified by several 

 physicists ; the form used in the most modern instruments is that due to 

 Glan, which is made as follows : A symmetrical 

 rhombohedral crystal, Fig. 229, is selected, and the 

 two faces are ground down parallel to each other 

 and perpendicular to the optical axis of the crystal ; at 

 right angles to these surfaces the prism a b c d is cut 

 out, and split along the direction b d ; the faces of the 

 prism and the surfaces at bd are then polished and 

 the two parts cemented together along the surface b d. 

 In this prism the field of vision is perpendicular to 

 and symmetrical with the optical longitudinal axis of the prism, and reflection 

 from the inclined surfaces as occurs in the ordinary form of nicol prism is 

 eliminated. 



Original Form Of Polariscope. Let there be two nicol prisms 

 arranged with their two optical axes in one and the same straight line, and let 

 a ray of light pass in the direction shown in Fig. 230, and as described in the 

 preceding paragraph through the prism on the left, referred to as the polarizer ; 

 and thence on to the similar prism on the right, referred to as the analyser. 

 Dependent on the relative positions of the prisms all, a part, or none of the 

 light will reach the eye of an observer looking from right to left towards the 

 source of light ; if the prisms are set so that their principal axes are parallel 

 a maximum of light passes ; if set so that their principal axes are not at right 

 angles to each other, none of the light passes. This position is referred to as 

 crossed nicols ; between these two positions a portion of the light passes. 



FIG. 230. 



Now let the two nicols be crossed, then no light reaches the eye of the 

 observer ; if, now, an optically active material be interposed between the two 

 nicols so that the plane in which the polarized light vibrates is rotated, a 

 portion of the light passes, and to obtain the effect of total darkness the 

 analyser must be rotated about its longitudinal axis through an angle equal in 

 magnitude, but opposite in sign to that through which the plane of polariza- 

 tion has been rotated by the optically active material ; now if to the analysing 

 prism a pointer be attached, which indicates zero on a scale at the position of 

 crossed nicols, then the angle through which the prism has to be rotated to 

 again produce the position of total darkness measures the rotation due to the 

 optically active material. 



The first polariscopes constructed by Biot 2 actually obtained their reference 

 point in this way ; but the position of total darkness is not one which can be 

 accurately determined, and other devices are described below. 



429 



