of Refraction in Selenium and Bromine. 55 



of transmission lies near the miJdleof the luminous spectrum. 

 Bromine would no doubt prove equally interesting, but is by 

 no means a pleasant substance to handle. 



I should hardly have ventured to come before the Society 

 ■with these very rough experiments^ had I not hoped that 

 some who possessed the instrumental means would be induced 

 to make the observations. For my own part I am more than 

 content with the humble though somewhat laborious office of 

 computer between mathematical investigator and observer. 



Table I. — Comparison of Sulphur and Selenium. 



Selenium. 



Density. 



Lower 

 index. 



/Xa- 



Upper 

 index. 



Arc. 



0A. 



Lower 

 limit. 



V 



v-1 



d ' 



Shortest 

 wave- 

 length. 

 \k. 



4-3 



2-653 



2-980 



44° 9' 0" 



2-3976 



32502 



5295-7 



Sulphur 



*d. 



//A. 



^E- 



0A. 



V. 



v-l 



d ' 





d' 207 

 d" 1-98 



j 1-9024 



1-9527 



20°32'0" 



1-8610 1 



41648 

 43535 





* d' is the density of native sulphur. 

 d" ,, „ of that from fusion. 



Thus we have the following emi)irical relation. If E be 

 the equivalent of sulphur, and v its limit of refraction, 



''^E = 13326, 



d' 



v-l 

 d" 



= 13932, 



Again, we have 



Orthobromotoluine, 



v-l =5299, 



Bef. equiv. Benzyl chloride^^- E = 5941-7, 



do. do. Chlorotoluine 



d 



v-l 



d 



E = 5907-2, 



