1877.] 



Refractive Indices of Glass. 



293 



It is interesting to remark that the curve representing fx in terms of 



t has a point of inflexion between C and D. An examination of the 



deviations from calculation for several glasses shows that probably all 

 glasses exhibit a similar point of inflexion, the flints lower in the 

 spectrum or in the ultra red, and the crowns nearer to the middle of 

 the visible spectrum. This fact may be of importance in the theory of 

 dispersion when a detailed theory becomes possible ; at least it is im- 

 portant as showing how unsafe it would be to calculate fi for very long 

 waves or ultra- violet waves from any formula of three terms. 



The following Tables of results need little or no further explanation ; 

 the first line gives the refractive indices finally obtained and regarded as 

 most probable, the second line gives the values of jll from the formula of 

 three terms 



^-l=a{l+6^(l4-ca?)}, 

 and the last gives the mean of the actual observations. 



Hard Crown. — Two specimens of this glass were examined. 



a. A prism from the three angles of which determinations were obtained, 



density 2-48575. 



«=0-523145. 6=1-3077. c= - 2-33. 





A. 



B. 



C. 



D. 



E. 



b. 



Most probable value of fi 





1-513625 

 1-513624 



1-514568 

 1-514560 



1-517114 1-520331 



1 



1-517099 1-520327 



1-520967 

 1-520966 



Mean of observed values 



1-511755 



1-513624 



1-514571 



1-517116 'l-520324 



j 



1-520962 







P. (G.) 



a. 



h. 



H r 





Most probable value of p 



1-523139 1-527994 

 1-523145 1-528003 



1-528353 

 1-528362 



1-530902 

 1-530906 



1-532792 

 1-532789 



Mean of observed values 



1-523145 1-527996 



1 



1-528348 



1-530904 



1-532789 



