Jr. E. Wright — Bi-quartz Wedge Plate. 395 



most commercial saecharimeters in order that white light may be 

 used, taking advantage of the fact that the rotatory dispersion 

 of sugar solutions is approximately that of quartz. According 

 to the scheme of fig. 3, plane polarized white light emerges 

 from the polarizer P, passes through the sugar solution T, the 

 different wave lengths being rotated through different angles 

 and thence through the quartz compensating system Q, where 

 they are again united and reduced to a common plane of 

 vibration. — that of the original polarizer. ' The quartz com- 

 pensating system is inserted until the field when observed 

 through the analyzer appears totally dark. As in the preced- 

 ing case, this condition is/verified with the greatest sensitive- 

 ness by inserting a bi-quartz plate wedge W. 



Up to this point, the advantages offered by the bi-quartz 

 plate wedge are mainly those of simplicity, both in construc- 

 tion and in manipulation, and of greatly decreased cost, without 

 any corresponding sacrifice of accuracy or sensitiveness. Its 

 sensitiveness is adjustable within any limits likely to arise in 

 usual polarimetric work ; it is equally adaptable to the mono- 

 chromatic and to the quartz compensating systems ; it is 

 possible with it to do away in considerable part with the photo- 

 metric principle by the use of Landolt's bands ; and finally, 

 the line of division between the hemispheres is so narrow as to 

 be practically invisible as an independent line. 



Furthermore, in so far as it avoids the small prism of the 

 Lippich polarizing system, it also avoids an error which is 

 inherent in this system, and incidentally also in the Bates 

 system, due to the loss of light in one half of the field in passing 

 into and out of this superposed prism. * In his analysis of the 

 problem, Bates has shown that the small angle 8 between the 

 normal to the principal plane of the analyzer and the bisector 

 of the angle. between nicols of the Lippich polarizer, can be 

 allowed for by a slight shift of the zero of the quartz compen- 

 sating scale for which provision is made in his apparatus. A 

 slight additional correction of the same character made at the 

 same point will also serve to correct for the loss of light by 

 reflection at the end surfaces, and by absorption within the 

 small prism. Supposing this to amount to 10 per cent of the 

 total intensity, the situation can be summed up as follows : 

 If a be the angle between the two prisms of the Lippich polar- 

 izer, I the intensity of the light from the large polarizer, then 



* It will be recalled that the Lippich system consists, either for monochro- 

 matic light or with the quartz compensator and white light, of a large nicol 

 in front of which is placed a small nicol covering one-half the field of the 

 former. The plane of polarization of this nicol makes a small angle with 

 the plane of the large nicol. In the matter of the loss of light at the surfaces 

 and by absorption within prisms, the system is therefore not symmetrical 

 with respect to the line dividing the fields. 



