372 



Messrs. du Bois and Rubens on Refraction 



Now n loses its physical meaning as soon as it ceases to be 

 constant ; and we nave given the variable values only to show 

 that they can be of no further use. A deviation of the metals 



Table II. (" Red/' Light). 



30°. 



40 c 



50°. 



55 c 



60°. 



65°. 



Prism Fe III. ; /2 = 25"'7 ; n=3'06. 



a, obs. ... 

 a, (SNELL.) 

 n, calc. ... 



51-7 



667 



733 



98-5 



1073 



1257 



53-0 



64-0 



74-7 



92-8 



106-4 



125-3 



{301 



315 



3-02} 



3-20 



3-08 



307 



o 



o 



o 



o 



o 



o 







93 



12-0 



144 



15-4 



163 







94 



124 



145 



15-5 



16-4 



— 



-o-i 



-01 



-01 



-01 



-01 



153-9 

 152-0 

 309 



im, integr. ... 

 i m ( Snell.) .. 

 Diff. 



o 



171 

 172 

 -01 



Prism Co III. ; £ = 23"-6 ; n=3 10. 



47 



61-2 



72-2 



951 



1130 



124-0 



49-7 



599 



700 



869 



996 



117-2 



{ 300 



3-15 



317} 



3-32 



3-42 



o 



3-24 



o 



o 



o 



o 



o 







9-2 



11-8 



140 



14-9 



15-7 







9 3 



11-9 



14-3 



15-3 



16-2 



— 



-0-1 



-01 



-0-3 



-0-4 



-0-5 



152-4 

 1426 



328 



16-4 



370 



-0-6 



Prism Ni III. ; £ = 27""7 ; n = l"93. 



a (Snell.) ... 

 n, calc 



i m , integr.... 

 im (Snell.) .. 

 Diff. 



27-3 



31-3 



36-8 



51-0 



61-2 



84-4 



258 



31-9 



38-2 



48-6 



567 



67 9 



{ 1-98 



1-91 



1-90} 



1-98 



202 



2-20 





o 







o 



o 



o 







14-7 



19-2 



231 



24-8 



26-2 







150 



19-5 



234 



25-1 



266 



— 



-03 



-0-3 



-0-3 



-0-3 



-0-4 



114-4 

 839 

 2-35 



273 



28-0 

 -0-7 



from the sine law is thus shown to exist, but to become 

 sensible only for inclinations above say 40°. We believe that 

 the nature of this deviation will be better understood if the 

 behaviour of each metal be compared with that of an ideal 

 transparent substance, which we assume to exist for this pur- 

 pose, and to possess a refractive index equal to the mean of 

 those found for the metal at inclinations of 0°, 30°, and 40°, 

 which are equal within the limit of experimental errors. 



