Errors in Methods of Measuring Rotary Polarization 5 



In these methods there is used as a measure of the rotation the 

 displacement of either a black space or a clear space in a spec- 

 trum. The former is produced by a quartz plate cut at right 

 angles to the optic axis and an analyzing nicol ; the latter, which 

 was first used by A. Schmauss,^ by an additional plate of selenite, 

 Avhich gives a channelled spectra with the interference bands 

 absent for those wave-lengths whose planes of vibration coincide 

 with one of the principal axes. From space to space in these 

 methods is a rotation of 180°, and the maximum sensibility 

 seems to be obtained with about 1.5 mm. of quartz between the 

 nicols. The working width of the space thus obtained depends 

 upon the observer, but is assumed to be about 40 /x/u. Both 

 methods are essentially photometrical since the position of the 

 space is determined by the condition that the light at equal dis- 

 tances on each side of the central region be of the same inten- 

 sity. Even if the clear space is located by having an equal num- 

 ber of the interference bands on each side of the central region, 

 the process is nevertheless photometrical because the bands do 

 not entirely disappear at any portion of the space, and the bands 

 at which the observer ceases to count must be of equal intensity. 



When either of these methods is used to measure the rotation 

 in the edge of an absorption band, there is a shift of the space 

 referred to, and consequently an apparent rotation of the light 

 due to the absorbing substance. Consider, for example, the dark 

 space method. Let the space be produced by 4.5 mm. of quartz 

 and the direction of vibration of wave-length 589.6 yu/ti be exactly 

 at right angles to its direction in the analyzing nicol. Under 

 this condition S', fig. 3, is the luminosity curve of the sun as it 

 reaches the eye; and F' the same curve modified by the insertion 

 into the path of the light of a i cm. length of the alcoholic 

 fuchsin solution 0.000024. The ordinates of S' are obtained by 

 resolving the direction of vibration, giving it its relative inten- 

 sity from curve S, fig. 2, along the direction of vibration in the 

 analyzer. F' is obtained by giving each ordinate of S' its relative 

 transmission from curve F, fig. 2. It will be observed from the 

 fact that the luminosity curve of the sun is not a horizontal 



»A. Schmauss, Antt. d. Phys. 2, p. 280. 1900. 



259 



