434 Prof. R. W. Wood on the Selective Dispersion of 



at various temperatures is very accurately known. As it 

 seemed desirable to work with very low vapour densities, 

 the interferometer method was the only one well adapted to 

 the work, and I ordered from Herseus a tube of fused quartz 

 10 cm. in length, terminated with end plates of the same 

 material, which had been ground and polished, and were 

 fused to the ends of the tubes. This tube is shown in fig. 1 

 of my paper on the selective scattering, &c. of resonating gas 

 molecules (Phil. Mag. 1912, vol. xxiii. p. 689). It contained 

 a drop of mercury and was very highly exhausted and sealed. 

 For the ultra-violet interferometer I had Mr. Petitdidier, 

 of Chicago, prepare for me a plane-parallel plate of white 

 fluorite, as it was feared that the double refraction of quartz 

 would give trouble. Fused quartz would perhaps answer 

 the purpose if it could be obtained free from striae, but as it 

 was my intention to carry on other investigations with the 

 instrument in the remote ultra-violet, I decided upon fluorite. 

 Some difficulty was found in getting a good deposit of silver 

 on the plate, and a cathode deposit of gold was accordingly 

 used. The back mirrors of the Michelson interferometer 

 were of speculum metal, and no compensating plate was 

 used. The quartz tube was introduced into the optical path 

 of the instrument in which the compensator is usually placed^ 

 and the two end plates which closed it very nearly equalized 

 the two paths, as their combined thickness was very nearly 

 equal to that of the fluorite plate. It proved to be quite a 

 problem to heat the tube, without at the same time heating 

 the instrument, or the air in its vicinity, for the former 

 would have caused a slow drift of the fringes, and the latter 

 a fluttering confusion of them, either of which would have 

 made photography impossible. The sodium tubes in the 

 earlier work gave very little trouble, as it was necessary to 

 heat only the central . portion of the tube, which was done 

 by an electric current. In the present case it was of course 

 necessary to have every part of the tube, including the end 

 windows, at the same temperature, and it seemed at first 

 impossible to do this and at the same time keep currents of 

 warm air out of the optical paths of the instrument. The 

 problem was finally solved in a very simple manner. An 

 air-bath was constructed of the form shown in fig. ± } out of 

 two large brass tubes, one fitting within the other with a 

 clearance of about a centimetre all around. Two small tubes 

 were soldered into the ends of the outer tube, one of which 

 was put in communication with an aspirator pump, and the 

 other with a glass tube which was heated by a Bunsen burner 

 turned down low. The temperature of the inner chamber 



