CANE SUGAR. 



The Laurent Half Shadow Device. 6 The Laurent half shadow 

 polariscope obtains its end point in a manner quite different to the instrument 

 described above. Between the polarizing and analysing nicol of ordinary con- 

 struction, and close to the former, is interposed a thin plate of active quartz, 

 which is cut parallel to the optical axis of the crystal. A beam of light 

 entering such a plate perpendicular to its surface, is doubly refracted into two 

 beams, with vibration planes parallel, and perpendicular to the optical axis. 

 In such a system that ray which vibrates perpendicular to the optical axis has 

 its speed of vibration increased, and the thickness of the plate of quartz is so 

 taken that that ray vibrating perpendicular to the optical axis has gained half 

 a wave length on the wave vibrating parallel to the optical axis at the 

 moment they emerge from the quartz plate. In Fig. 28Jj. let the circle 

 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 I ; 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 1 9 and is now represented by the line o 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, symmetrically arranged with respect to the 

 optical axis of the quartz plate. The effect of this 

 is to obtain a field of vision exactly similar to that 

 described in dealing with the Jellet-Cornu apparatus, 

 and the remarks made there are applicable. It must 



be noted that the half shadow angle with this instrument can be varied, the 

 larger the angle the greater the intensity of the light, but the less the delicacy 

 of the apparatus. With this apparatus very dark-coloured materials can be 

 observed, but with a loss in the fineness of the reading. 



Horsin-Deon Polariscope. 7 This instrument is of different con- 

 struction to any of those previously described. The light passes through a 

 Jellet 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. This 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 spectrum. 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 instruments. 



432 



