1904.] 



The Optical Properties' of Vitreous Silica. 



205 



Focal Curve. — The curve shown (p. 204), is that for a thin doublet of 

 fluorite achromatised by vitreous silica (vitrified quartz). It will be 

 seen that the focal length of the combination is almost independent of 

 the wave-length. A list of focal lengths is also given (Appendix I). 



Measure of Error. — The approximate estimate of the error from 

 group deviations before referred to,* shows that of the 26 final 

 deviations : — 



In 6 of them the 



error is less than 0""667, 



„ 7 



,, ,, 1 *333, 



55 8 55 



2"-667, 



4 



55 * 55 



,, more „ 2"'667, 



„ 1 only 



is as much as 9" "5 14, 



the respective and corresponding variations of the index being — 



0-0000022 

 0-0000045 

 0-0000089 

 0-0000286 



As additional evidence of accuracy, it may be 

 mentioned that the first measurement of line D 

 was made with the larger prism in May, 1902, 

 the index being 1-4584772 



The other prism mentioned above, built up of four 

 separate small prisms of different meltings, 

 cemented and ground all together, was pro- 

 duced in April, 1903, and gave an index of ... 1*4584830 



The difference of the two readings being 0-0000058 



A similar comparison of measurements from both prisms of the 

 F line shows a difference of 0-0000103. 



The coincidence of the four lines given by the four small prisms, of 

 which the latter was built up was perfect, the definition being equal 

 to that of the first prism. The D Lines were in both cases seen 

 distinctly separated. 



Probable Accuracy of the Indices. — The prisms were comparatively 

 small and contained numerous bubbles.! For this reason the accuracy 

 of the indices is not so great as of those of ordinary quartz already 

 given.:}: But the indices of vitreous silica may be taken as correct 

 to the fourth decimal place, and it is believed that in almost all 

 cases the error does not exceed unity in the fifth place and in many 

 does not exceed five in the sixth. 



* Loc. cit. 



f This defect is not inherent in the material and will doubtless be eliminated as 

 experience is acquired. 

 % Loc. cit. 



