Magnetic Rotary Dispersion of Solutions 3 



of a similar tube filled with the solvent noted. Since the tubes 

 are the same length the difference between the two rotations is 

 the rotation, for the wave-length of light used, due to the pres- 

 ence of the dissolved substance. 



The source of light was the sun. The desired wave-lengths 

 of sufficient homogeneity were obtained by means of a spectral 

 system similar to that described by Doubt. ^ 



The instrument used in making the measurements was a sensi- 

 tive-strip spectropolariscope. ^ The polarizing system consists 

 of two thin pieces of Iceland spar mounted in a cell of a-mono- 

 bromonaphthalene and transmitting the ordinary ray. The smaller 

 of the two pieces covers but half of the field of the larger one, 

 and the angle between their planes of polarization can be varied 

 at will. Both the color and polarizing systems will be treated in 

 detail by the writer in a paper soon to be published. 



The conical pole-pieces of the electromagnet used tapered from 

 20 cm. to 5 cm. in diameter. With 23 amperes at 500 volts and 

 the pole-pieces separated 17 mm., a field of over 18,000 lines per 

 sq. cm. was obtained. This intensity was used with the litmus 

 and anilin blue solution, but was allowed to drop to 15,000 lines 

 in studying the fuchsin and cyanin. This was sufficiently near 

 saturation to prevent the slight fluctuations of the current inter- 

 fering with the setting of the polariscope. 



A star-shaped brass frame, fig. i, rotating on pivot bearings, 

 was rigidly supported between the poles of the magnet. The 

 tubes rested in the V-shaped slots and were held in place by 

 spring-back clips. A slight rotation removed one tube from the 

 path of the light and substituted another. The tubes were i cm. 

 in length and closed with cover-glasses each 0.2 mm. in thickness. 



Since we compare one tube with another it is necessary that 

 their optical paths be the same. Factors tending to make their 

 paths unequal may be introduced, first, by the tubes not being of 

 the same length ; second, the brass frame failing to keep their 

 axes parallel ; third, the cover-glasses not being of the same thick- 

 ness. These difficulties were all eliminated by surrounding the 



»T. E. Doubt, F/iil. Mag. Aug., 1898. 

 »D. B. Brace, Ibid. Jan., 1903. 



267 



