Effect of the Ultra-violet Spectrum, 

 cln 



293 



the furthest points 3- becomes very great for the large 



values of n, where the curve is nearly parallel to the axis 

 of fjb. Beyond this point (\ = 200 (jl/jl) the connexion between 

 A and n is unknown, but quartz is said to cut off all waves 

 shorter than \= 18 5 ja/j, ; this, however, is not in agreement 

 with the results obtained by the electrical method. Rubens' 

 curve is drawn for the ordinary ray. but after refraction 

 through quartz the extraordinary ray is not very far sepa- 

 rated from the ordinary, and is, moreover, very feeble in its 

 effects. The absorption by quartz undoubtedly increases as 

 the waves get shorter, as is also the case with the absorption 

 by air. 



The apparatus used to measure the electrical effects of the 

 different parts of the spectrum was as follows. 



The source of light was an oscillatory electrical discharge 

 passed between two aluminium terminals; the length of the 

 spark-gap was i mm. and the terminals about 1 mm. in 

 diameter. The spark-gap was connected to the secondary of 

 an 8-inch Ruhmkorff coil in parallel with a leyden-jar. The 

 current through the primary was about 3 amperes, and a 

 hammer interrupter was used. This source of light, besides 

 being very rich in ultra-violet rays, was of very steady 

 intensity. 



Fi<r. -2. 



ft 6 



7/77776 



The spectrum icas formed as in fig. 2. The spark-gap S 

 was placed in a metal box A, this, as usual, being necessary 

 to shield the rest of the apparatus from induction effects. 



The light entering the collimator tube through a vertical 

 slit at B was rendered parallel by the first quartz lens D and 

 fell upon the prism E ; this prism was of quartz having 

 angles of 60 degrees. With such a prism working at 

 minimum deviation no trouble can be experienced from 



