Dispersion in Relation to the Electron Theory. 241 



The condensing lens C was mounted upon an electrically- 

 driven tuning-fork of frequency 30. The lens was thus 

 maintained in vibration in a vertical plane, and the light 

 distributed more uniformly over the field of the polarizer. 

 The use of a lens vibrating harmonically, though possibly 

 open to theoretical objections, was in actual practice found 

 to render the results much more consistent and reliable. 



The projection lens was a quartz-fluorite combination o£ 

 32 cm. focal-length, giving exceedingly good achromatism 

 through the ultraviolet spectrum. It was corrected for 

 spherical aberration at *300//,. 



A suitable source of light of the required intensity for this 

 type of experiment is difficult to find. A quartz-mercury 

 lamp possesses many advantages, but lines of the requisite 

 intensity are not very evenly spaced along the spectrum. 

 Probably the best source is a quartz-tube amalgam lamp, 

 but in the absence of the latter and after trials with a simple 

 mercury lamp, it was decided to use the iron arc as the 

 source. The difficulty here is to obtain sufficient steadiness. 

 Owing to the relatively large current which must be used, 

 the negative pole melts, a circumstance which, it was found, 

 prevents the use of a rotating arc, excellent as this is for 

 ordinary spectrographic work. Iron-magnetite, and iron- 

 cassiterite arcs were also tried, but it was ultimately found 

 that the best work could be done with a simple iron arc 

 arranged as in fig. 2. The negative electrode is vertical and 



Fiar. 2. 



adjustable while the positive is horizontal and connected to 

 the circuit at both ends through equal manganin resistances, 

 R 1? R<j. A current of 7 to 8 amperes was used with poles 

 1*5 cm. in diameter. With this arrangement, once the poles 

 become thoroughly hot, the molten iron from the negative 

 Phil. Mag. S. (). Vol. 31. No. 1 83. March 1 916. S 



