WOOD. — ANOMALOUS DISPERSION OF SODIUM VAPOR. 381 



prepared the apparatus for showing the anomalous dispersion of the 

 vapor, which is one of the most beautiful optical experiments that I 

 know of, can be set up in Hve minutes. The former method which I de- 

 scribed was rather troublesome, as it involved the use of a hydrogen 

 generator and drying tubes, and it was often difficult to avoid the forma- 

 tion of smoke in the tube. The vacuum tubes make the experiment as 

 easy to perform as the simplest of the laboratory demonstrations. A 

 combustion furnace is not necessary, two or three Bunsen burners turned 

 down low answering every purpose. The amount of dispersion can be 

 regulated by turning the flames up or down. A full description of the 

 disposition of the apparatus will be found in my first paper.* In brief, it 

 consists of a horizontal slit on which the crater of an arc lamp is focussed. 

 If a Nernst lamp is available it may advantageously be substituted for 

 the illuminated slit, the glower being mounted in a horizontal position. 

 The light from the slit is made parallel with a lens, and after traversing 

 the sodium tube is focussed on the slit of a small spectroscope by means 

 of a second lens. It is almost as satisfactory to use a single lens passing 

 the convergent beam through the dispersion tube. 



If it is desired to show the anomalous spectrum, a low power eye-piece 

 is substituted for the spectroscope. The appearance of this spectrum 

 is shown by a colored plate illustrating the earlier paper. In the present 

 work it was desirable to have a normal spectrum, and the spectroscope 

 was replaced with a collimator and plane grating. The light from the 

 grating entered a camera made of a long wooden box, provided with an 

 achromatic telescope lens of 2 meters focus. The spectrum, which was 

 received on a plate of ground glass, was about 25 cms. long and 1 mm. 

 wide. Its position was accurately recorded on the ground glass by 

 means of a narrow pencil mark ruled along its centre, the ground sur- 

 face having been sponged over with oil to increase its transparency. 

 The burners of the furnace were now lighted and turned down very low, 

 so that the tips of the flames fell a little short of the tube. As soon as 

 the temperature of the tube became constant, the curved branches of the 

 dispersed spectrum were outlined on the glass with the pencil. The 

 burners were then turned up a little and the deviations of the oppositely 

 curved branches increased, their new positions being recorded in the same 

 manner. A number of records were made in this way, and the tube was 

 then allowed to cool off. The spectrum straightened, and was found to 

 occupy the same position on the plate as before, showing that no displace- 



* Wood, The Anomalous Dispersion of Sodium Vapor. Proc. Roy. Soc : also 

 Phil. Mag., 1901. 



