422 Prof. Wood on the Magneto- Optics of Sodium Vapour 



The Bright Line-Spectrum produced by Magnetic Rotation. 



Attention has already been drawn to the remarkable bright 

 line-spectrum which presents itself when the nicol prisms are 

 crossed. This spectrum, which at first could only be seen 

 with the greatest difficulty, was finally obtained of such 

 brilliancy that it could be photographed with the 14- foot 

 concave grating. A good vacuum was found to be the most 

 essential condition. In the earlier work with tubes sealed 

 off from the pump, the pressure due to the liberated hydrogen 

 was probably responsible for the faintness of the lines. The 

 density of the sodium vapour must be just right. If it is 

 too rare, the rotation is insufficient to bring out the lines, if 

 too great the absorption weakens the spectrum. When the 

 conditions are exactly right, the lines are almost as bright and 

 numerous as the metallic lines in the arc. Photographs of 

 the complete spectrum were made on trichromatic plates, 

 made by the Cramer Dry Plate Co. of St. Louis. These 

 plates were found to be sensitive up to the red lithium line 

 (\= 6705) . An exposure of fifteen minutes was sufficient with 

 a small concave grating of about a metre radius. Two of these 

 photographs are reproduced natural size on Plate Y. fig. 4. 



I have never observed a doubling of any of these lines 

 such as occurs in the case of T) x and D 2 . So great is the 

 quantity of the light transmitted by the second nicol, that a 

 brilliant orange- coloured image of the crater of the arc 

 appears on the slit of the spectroscope the moment the current 

 is turned into the magnet, notwithstanding the fact that the 

 nicols are crossed. 



Inasmuch as it seemed desirable to investigate these lines 

 under higher dispersion and determine their wave-lengths 

 as accurately as possible, the apparatus was transferred to the 

 concave-grating room. 



The Ruhmkorff magnet was abandoned at this point, as it 

 was found that an exposure of four or five hours would be 

 necessary, and the magnet became dangerously hot in half 

 an hour, when fed with the necessary current. In its place 

 a very large magnet, built by Professor Rowland for the 

 study of the Zeeman effect, was used. This magnet could be 

 fed with a 110-volt current without resistance, and operated 

 continuously for any necessary length of time. 



A steel tube nearly 3 cms. in diameter could be used with 

 this magnet, which increased in no small degree the amount 

 of light available. After a number of failures, a very satis- 

 factory negative was secured with the 14-foot grating show- 

 ing the rotation spectrum and the absorption spectrum side 



