1904.] 



The Optical Properties of Vitreous Silica. 



201 



When extracting data from the earlier of the two papers by Mr. Ellis* 

 already mentioned, I found that he had there advanced evidence — 

 though he seems hardly to have considered it conclusive — that the 

 •difference between the Greenwich declination and horizontal force 

 ranges in years of many and few sunspots was not the same in different 

 seasons of the year. Converted into exact mathematical language, this 

 would imply the variability of b throughout the year. If I had been 

 .aware at the time of Mr. Ellis's remarks on this point I should have 

 referred to them in my preliminary notef on this subject. 



* The Optical Properties of Vitreous Silica." By J. W. Gifford 

 and W. A. Shenstone, F.RS. Received January 12, — 

 Head March 3, 1904. 



The properties of vitreous silica suggest that it is not unlikely to 

 play an important part in optical work. Its composition is definite, 

 that is to say, it is not liable to those minute variations which make 

 it impossible, we believe, to produce with certainty two meltings of 

 glass which exhibit no sensible difference in their optical properties 

 when tested by a first-rate spectrometer. Hardly any corrosive fumes, 

 except those of fluorine and hydrogen fluoride, attack silica, and it 

 is indifferent to most ordinary solvents. It is as transparent to ultra- 

 violet radiations as quartz, but is not doubly refracting like that 

 substance. And though it is a little difficult to prepare vitreous 

 silica in large masses, this difficulty can be surmounted, and the 

 supply of the substance is not limited like that of fluorite. In short, 

 vitreous silica places at our disposal a really standard glass. The 

 refractive index of the new glass is low, it approaches that of fluorite. 

 Its dispersive power is sensibly greater than that of quartz. 



The measurements given in this paper were made with a prism 

 having faces 41 mm. high by 32 mm. wide, and angles of 60° 

 .approximately. The mass of silica from which this prism was cut 

 was made under the supervision of one of us in conjunction with 

 Mr. H. G. Lacell, to whom our thanks are due. As it was our object 

 to produce a standard substance for optical work, no care was spared 

 at this stage. In making the mass of silica the spectroscopic traces 

 -of lithium and the traces of sodium which occur in quartz were burnt 

 •out as completely as possible in the oxy-gas flame. The prism itself 

 was built up from many hundreds of fine rods of vitreous silica, 

 prepared specially for this purpose by a process which has been 



* ' Phil. Trans.,' for 1S80. 



f ' Eoy. Soc. Proc.,' vol. 71, p. 221. 



