270 E. S. Lav sen — Relation between the Refractive Index 



The specific refractivity of the various glasses and crystals 

 studied have been computed according to both the formula of 

 Gladstone and Dale and that of Lorentz and Lorenz, and the 

 results are tabulated in Tables 4, 5 and 6. Table 4 gives the 

 data for the feldspar series. The third column gives the value 



Table IV. 



Glass 

 Ab 



Ab 2 An 1 __. 

 AbiAii!... 

 AbiAn 2 __. 

 AbiAn 5 .-. 

 An 



Feldspar 



Ab .. 



Ab 2 Arij__. 

 AbiAiij... 



AbiAn 2 



Ab 1 An 5 -.. 

 An' 













o 

 o 



o 

 o 



M 

 II 





o 

 o 



N Na 



D 



M . 

 II 



K' 2 



u 





= M 





T-l C3 



K' 2 



u 









1 



« 





1 



M 



1 



k i 



1 



+ 





1 



M 

































£ 







M 





M 





K 



& 





M 



1-4890 + 



2-382 



•2053 



•2053 



o-oo 





•12117 



•12117 



o-oo 



1-5166 



2-483 



•2081- 



•2080- 



+ 0-05 





•12176 



•12161 



+ 0-12 



1-5307 + 



2-533 



•2095 



•2094 



+ 0-05 





•12208 



•12185 



+ 0-19 



1-5452 



2-591 



•2104 



•2105 



-0'05 





•12206 



•12206 



o-oo 



1-5600 



2-648 



•2115 



•2119 



-0-19 





•12211 



•12228 



-0-14 



1-5755 



2-700 



•2132- 



•2132- 



o-oo 





•12248 



•12248 



o-oo 



1-534 



2-605 



•2050 



•2050 



o-oo 



+ 0-15 



•11933 



•11933 



o-oo 



1-549 



2-660 



•2064 



.2075 



-055 



+ 0-82 



•11958 



•12010 



-0-43 



1-558 



2-679 



•2083 



•2088 



-025 



+ 0-58 



•12034 



•12047 



-0-11 



1-568 



2-710 



•2096 



•2100 



-020 



+ 0-38 



•12073 



12083 



-0-08 



1-577 



2-733 



•2111 



•2112 



-0-05 



+ 0-19 



•12125 



•12119 



+ 0-05 



1-587 



2-765 



•ais 



J3 



•2123 



o-oo 



+ 0-40 



•12154 



•12154 



o-oo 





+ 1-52 

 + 1-82 

 + 1-45 

 + 1-10 

 + 0-71 

 + 0-77 



of K 2 as computed from the simpler formula, the next column 

 the value of K\ from the end members of the series, on the 

 assumption that it is an additive function. The column headed 



K — K' 



1 — L .100 gives the percentage by which K a differs from the 



computed additive value. This difference is within the limits 



of error for the glasses but is a little greater than the probable 



error for the feldspars. 



K — K" 

 The column headed ' -.100 gives the percentage by 



which the specific refractivity of a crystal differs from that of 

 a glass of the same composition. For the feldspars and their 

 glasses this difference is always positive and is somewhat 

 greater than the probable error. 



In the last four columns of the same table, corresponding 

 values computed from the n 2 formula are given. The deviation 

 from the additive relation is within the limits of error for 

 both the glass and the crystalline series as in the former case, 

 but the percentage of difference between the constant for a 



