THE POLARISCOPE AS APPLIED TO SUGAR ANALYSIS. 



The Jellet-Cornu Half Shadow. The optical parts are shown 

 in Fig. 2/J.2, the reference being as in Fig. 



The Landholt-Lippich Half Shadow. The optical parts are 

 shown in Fig. 2!j.3. 



The Landholt-Lippich Triple Shade. The optical parts are as 

 for the half shadow, except forthe polarizer, which is shown in Fig. 2!j.5 two small 

 nicols of exactly similar construction and adjustment are used so that the field 

 is divided into three parts ; in all positions except when 

 equality is obtained there appears a black band in the centre 

 of the field, bounded by a light portion on either side, or 

 vice versa ; this device is of great sensitiveness, but, owing to 

 the great delicacy of the adjustment of the reference point 

 device, is not to be recommended for use where access to an 

 optical instrument expert is difficult. 



The Laurent Half Shadow. The optical parts 

 of the early Laurent instruments are shown in Fig. 244'-, 

 these instruments used the sodium light and compensated by 

 rotation of the analyser; the normal weight was 16-19 grms. 

 in 100 true cc. FIG. 245. 



The modern form of the Laurent instruments is shown in Figs. 2!j.6 and 

 247 ; these instruments are now made so that by an adjustment either the 

 sodium light or white light may be used ; in the former case, compensation is 

 effected by rotation of the analyser, and in the latter by quartz wedges. 



Laurent instruments are constructed in which the lamina of the quartz 

 securing the half shadow device is inserted as a circular disc, concentric with 

 the field of vision. 



Polariscope Lamp. When ordinary white light is used it is 

 essential that it be as bright as possible ; an ordinary duplex paraffin lamp, 

 carefully trimmed, gives good results ; better results still are obtained by an 

 incandescent mantle with gas or alcohol as the source of light, and the 

 acetylene lamp gives perhaps the best illumination. The incandescent electric 

 light is also convenient, but requires the interposition of a ground glass disc. 

 Some years ago lamps supplied from the makers of polariscopes were fitted 

 with a metal chimney, in which was a bull's-eye lens. This is not only 

 unnecessary, but even objectionable; it is, however, convenient to enclose the 

 source of light in an asbestos chimney provided with a narrow opening, 

 towards which the polariscope is directed ; this arrangement is to shield the 

 eye of the observer from extraneous light. The distance of the lamp from the 

 instrument is of importance, and in modern instruments it is from 15 cm. to 



439 



