THE POLARIMETER 



479 



represent the diaphragm opening, covered as to one half by the quartz plate, 

 and let the optical axis of the plate be represented by the line o b ; let o a 

 represent the amplitude of vibration and the plane of polarization of the light 

 coming from the polarizing nicol. On meeting the quartz plate this ray is 

 resolved into two rays, o b and o e, parallel and perpendicular to the optical 

 axis of the quartz plate ; on emerging from the quartz plate the ray o e has 

 gained half a wave length on the ray o b, and is now represented by the line 

 d. These two rays can be compounded into the ray o c, precisely as if the 

 field of vision was illuminated by the rays o a and o c, sjTnmetrically arranged 

 with respect to the optical axis of the quartz plate. The effect of this is 

 to obtain a field of vision exactlj- similar to that described in dealing with the 

 Jellett apparatus. The Laurent half-wave plate may take the form of a 

 central disc or of a ring. In the former method one position of unbalance 

 appears as in Fig. 294, a tripartite field shown in Fig. 295 as unbalanced 

 obtaining in the latter case. 



Poynting Half Shadow Device.^ — This consists of a plate of quartz cut 

 perpendicular to the optic axis, and of which one half is very slightly reduced 

 in thickness. As a means of obtaining variable sensibiUty it is suggested 



Fig. 293 



Fig, 294 



Fig. 205 



that a scheme involving the principle of the Soleil-Duboscq compensator 

 (<7 u.) could be used. A simpler means yet consists of a cell filled with some 

 active material, the horizontal depth of which is reduced as to one half by 

 inserting in the cell a thin glass plate. The effect produced by this device 

 is similar to other half shadow contrivances. 



Horsin-Deon Dmc^.^*— This instrument is of different construction 

 from any of those previously described. The light passes through a Jellett 

 prism, and then through a plate of dextro-rotatory quartz rather more than 

 4 mm', thick ; the effect of this is to produce a blue field on the left and a 

 pale yellow field on the right. The compensator is a wedge of levo-rotatory 

 quartz, behind which is placed a disc of levo-rotatory quartz, the effect of 

 which is to produce a final tint rather darker than the sensitive tint of the 

 colour polariscope. The field of view of this instrument in positions remote 

 from the zero position is that one half is colourless, and the other coloured in 

 all colours of the spectnmi. Near the zero position the colourless half 

 becomes tinted before the other half loses its colour ; at the zero position, 

 the field of view is a uniform field, similar to that of the half shadow instru- 

 ments. 



The Lippich Half Shadow Device. ^'^— This device obtains its half shadows 

 by the interposition of a small Nicol prism between the polarizer and the 



