89S 



Magnetic rotatory power. 



DLslim-t periodic curves are ohtaiiicd. The liaiogeii elements occupy 

 the lopuu»st |ioiiils. 



Pliijsico-Vlicinlcal Luborator;/, J-'re.sidenr;/ Colltye, Calcutta. 



Physics. — "Fresnei.'s coi'/ficii'iit for JiijJit of (liferent colours." 

 (Second part). By Pvof. P. Zekman. 



(Gommuuicated in the meeting of May 29, 1915 ) 



A tirst series of experiments was made witli yellow, green, and 

 violet (4358) mercury light. As Fkksnkl's coefficient changes only 

 slowly with the wavelength, such a high homogeneity of the incident 

 light is unnecessary. With regard to the intensity of the light it is 

 even recommendable to work with a limited part of a continuous 

 spectrum. In a second series of experiments I therefore analysed 

 the light of an electric arc (12 Amp.) with a spectroscope of constant 

 deviation, which I had arranged as a nionochroraator by taking 

 away the eye-piece and replacing it by a slit. The nionochroraator 

 had been calibrated with mercury and helium lines. The prism stood 

 on a fable, which could be turned by means of a screw. Each 



