Effect of the Ultra-violet Spectrv. 



in. 



291 



ordinary ray in quartz the refractive indices corresponding 

 to different wave-lengths as far as X=200 pp. His results 



are shown in fig. 1. 



too 



?00 



400 



200 



Fig". 1. 









I 1 

 1 



























\. 











§ 































A 



~Fff/)CT/V£ 



/nds*. 



i-55 156 



1-58 



The ultra-violet spectrum has also been examined by the 

 bolometer, and Pfliiger * has by this means measured ap- 

 proximately the energy of the different parts of the spectra 

 of spark discharges between terminals of various metals. 

 For aluminium terminals and a fluorspar prism he reached a 

 limit of \=180 pp. His results will be mentioned later. 



A third and novel method of studying the ultra-violet 

 spectrum is based on the well-known fact that ultra-violet 

 light rays have a very destructive effect on various bacteria, 

 and Professor Dreyer has very kindly given me the following 

 account of experiments based upon this. Dr. Sophus Baugt 

 investigated the effect of various parts of the spectrum, and 

 showed that the bactericidal power commenced just about 

 the violet end of the spectrum, rose to a maximum at about 

 350 pp } fell to a minimum at 310 pp, and then rose very 



* Annalen der Physik, xiii. p. 890 (1904). 



t " Om Fordelingen af bakteriedrabende Stossler i Kulbuelysets Spek- 

 tmm." Meddel. fra Finsens medic. Lysinstitut, ix. 1904. p. 123. 



U2 



