244 Mr. S. S. Richardson on Magnetic Rotary 



As very satisfactory extinction was obtained with crossed 

 nicols, it was concluded that no appreciable quantity of 

 doubly-reflected light entered the spectroscope and no correc- 

 tion was deemed necessary. In introducing such corrections 

 on theoretical grounds only there is danger of over-correc- 

 tion, as absorption within the substance under test during 

 the double transit or a very slight want of parallelism 

 between the end-plates would prevent the reflected light 

 from entering the spectroscope-slit. 



From the observations on pure carbon bisulphide for the 

 D line, and at 20° C, using the value for Verdet's constant 

 obtained by Rodger and Watson * : 



S = -04347--0 4 737£, 



the change of magnetic potential on reversal of the field 

 (0 = 7*5 amperes) was found to be 13,322 cm.-gauss. 



Experiments with a Solenoid. — For liquids which can be 

 obtained in considerable quantities and which are sufficiently 

 transparent in long columns, a solenoid possesses several 

 advantages over an electromagnet. A much larger mag- 

 netic potential is easily obtained, with consequent increase 

 in rotation and dispersion ; and if the ends of the experi- 

 mental tube are allowed to project beyond the ends of the 

 coil, the method lends itself particularly to the determination 

 of absolute rotations!. For measurements of the rotary 

 dispersion the chief disadvantage is that the field is strictly 

 proportional to the current-strength, and it is therefore 

 necessary to keep the latter more nearly constant than when 

 using a magnet with iron cores brought nearly to saturation;, 

 and the large resistance of a solenoid leaves little room for 

 ballasting resistance unless a battery of an exceptionally 

 large number of cells is available. The heating of the coils 

 causes a rapid decrease in the current, but I find that with a 

 water-cooled coil the variation of current is very small after 

 the initial stage of the heating provided the flow of water is 

 steady, and with the aid of a small rheostat can be kept con- 

 stant to within 1 part in 1000 by occasional adjustment. A 

 diagram of the coil designed for use with a battery of 

 50 accumulators and for a current of 7 "5 amperes is given in 

 fig. 3. A brass tube B, 7*7 cm. in diameter, is mounted on 

 two ebonite uprights E, 16 cm. thick and 21 cm. high. The 

 tube is attached to the ebonite by means of two brass flanges, 

 the inner faces of which are lined with fibre. The coil, 



* Phil. Trans, A, 1895. 

 t Ibid. 



