1904.] The Optical Properties of Vitreous Mica. 



203 



although the prism consisted of no less than four distinct masses of 

 silica from four separate meltings. 



We do not know whether a similar result could be obtained in 

 the case of separate specimens of glass from different meltings. But 

 we think not, for we found on a trial that a compound prism of 



-58 mm.- 



Compound Silica Prism, actual size. 



Schott's borosilicate flint (No. 0'364), made from four pieces of glass 

 taken from different parts of the same melting, gave a much less 

 satisfactory spectrum than the compound prism made from four 

 separate meltings of silica. When the two prisms were compared in 

 the spectrometer, the D line given by the glass prism presented itself 

 as a bundle of six lines,* whilst that given by the silica prism was 

 normal, the D line being sensibly divided. 



Measurements of Refractive Index. 



Measurements of the refractive indices have been made with the 

 same goniometer, by the same method, and for the same wave-lengths 

 as those previously given in the case of fluorite, quartz and calcite,f 

 and the same tests have been applied. 



Errors due to the Method. — No one of the angles of the prisms differed 

 from 60° by as much as 1' of arc, and the error is therefore in the 

 tenth place of decimals.; 



* The measurements of the mean D line for the components of the glass prism 

 were :— Nos. 1 and 4 = 1-5757374, No. 2 = 1-5757179, No. 3 = 1-5757978. 

 f « Koy. Soc. Proc.,' February 13, 1902. 

 X Sea note by Dr. E. T. Q-lazebrook, loc. ext. 



