F. E. Wright— Bi-quartz Wedge Plate. 391 



Art. XLI. — The Bi-quartz Wedge Plate Applied to Polar- 

 imeters and Sajccharimeters ; by Feed. Eugene Weight. * 



In the preceding paper on the measurement of the extinction 

 angles of mineral plates in the thin section, the general theory 

 of the relative intensity of illumination of the field for different 

 positions of the nicols and also of the crystal section is devel- 

 oped. The different methods for crossing the nicols accurately 

 and also for determining the position of total extinction for a 

 crystal section are considered with respect to their sensitiveness, 

 and the conclusion reached that the best results are obtained 

 by dividing the field into parts in which the planes of polariza- 

 tion are inclined at equal but small angles (90-(f> of the general 

 formula) to the line of division between them (half shade 

 system), f For given conditions of illumination and sensitive- 

 ness of the observer's eye, there is always a certain angle (90-0) 

 for which the phenomena observed are most sensitive to slight 

 movements from the position of true extinction. To reach the 

 maximum efficiency of the half shade device, this angle (90-0) 

 should therefore be adjustable within the limits prescribed by 

 these conditions. 



The conditions for maximum sensitiveness have been worked 

 out for polarimetric purposes with sufficient care and accuracy 

 by a number of investigators, % and several instruments have 

 been designed in which high precision is possible, provided only 

 that light of a certain quality and intensity is available and 

 that the substance under investigation (sugar, for example) 

 permits just the right quantity of this light to pass through it 

 to fall within the rather narrow limits in which the observer's 

 eye is most sensitive. In practice, these conditions have proved 

 somewhat exacting, and considerably greater elasticity in the 

 adjustment of the optical system of the instrument with a view 

 to service under a greater variety of experimental conditions, 

 without loss of accuracy, is desired. Mr. Frederick Bates§ of 

 the Bureau of Standards has recently successfully designed an 

 instrument in which this feature has received attention. His 

 polariscope follows the Lippich system with the addition of 

 sets of gear wheels for revolving the large polarizer and analyzer 

 simultaneously in such a way that the angle of revolution of the 

 polarizer at every instant is twice that of the analyzer. He 

 has also demonstrated that the small angle (8) between the 



- The author is indebted to Dr. Arthur L. Day of this laboratory for several 

 important suggestions in the preparation of this paper. 



+ S. Nakamura, Centr. f. Min., 1905, p. 267-279; P. G. Nutting, Bulletin, 

 Bureau of Standards, iii, 249, 1906. 



X See Winkehnann. Hanclbuch der Phvsik (2), vol. vi, pp. 1362-3. 



§ Bulletin of the Bureau of Standards, iv, 461, 1907. 



