1 42 M. Rudbierg on the Refraction of the differently-coloured. ^ 



w'" n'" 



Ratio — r Ratio —rr 



Hence every colour in arragonite has a double refraction as 

 much greater as it is more refrangible. This result agrees 

 with that for rock crystal and Iceland spar ; and we may there- 

 fore conclude in general, that 



Each colour has its individtial double refraction as much 

 greater as its o'jo?i refrangibility is greater-* . 



By means of the preceding values of the indices n', n' and 

 «'", we may calculate the angle of inclination « of the optical axes 



n'""- — n"^ 

 by the formula sin^ I « = —7773 75- • as in the following 



table. 



Hence we see that in arragonite the inclination of the optical 

 axes diminishes contimiallyfrom the violet to the red light. 



Dr. Brewster gives for the true inclination of the optical axes 

 18° 18', calculated from the observed apparent inclination. 

 But as he has not given the value of this apparent inclination, 

 nor the index which he made use of to calculate the true in- 

 clination, it is impossible to compare his result with that of 

 ray experimentsf. 



Having several times measured the apparent inclination of 

 the axes by means of a plate with parallel faces cut perpen- 

 dicularly to the axis A, I found it a little more than 32°. To 

 make a comparison with this value, we must calculate the 

 apparent inclinations from the true inclinations as given in the 

 above table. This is easily done ; since we can now determine 

 the velocity of light in the direction even of an optical axis. 

 If we insert in the formulae, given at the beginning of this sec- 

 tion (p. 138), e" = 0, and e' = a, we obtain 



v" = v"^ = n'"' _ (w'"2 - n'') sin'2 ^ « 

 in which v' = v" = n'. 



* See our last Number, p. 6. t Sec the next Article. 



