1821.] Crystallized Bodies on Homogeneous Light, 173 



instance, and it, therefore, becomes necessary to ascertain the 

 values of / by direct measures. This is rendered easy by th^ 

 equation (e) which gives .) 



/=^. 



sin. e . iin. 6' 

 n . COS. (p 



We have only, therefore, to observe the inchnations of a plate 

 of known thickness, properly cut and adjusted to 45° azimuth, 

 which correspond to the alternate disappearances of the ordi- 

 nary and extraordinary images, at which points the values of n 

 ^^^ T> h^y h ^^- 5 computing then the values of 6, fl', and ^, and 

 substituting, we get the values of /, without detailing particular 

 experiments. The following table expresses the final result of a 

 great number of such measures : 



Colour. 



Extreme red .. 



Mean red. 



Mean orange . 

 Mean yellow . . 

 Mean green. . 

 Mean blue. . . . 

 Mean indigo . 

 Extreme violet 



mi$i 



The observations from which this table was calculated were 

 made indiscriminately on the maxima and minima of all orders. 

 Those of different orders were of course computed separately, 

 and found to agree without exception in giving the same values 

 of / within limits of error less than those to which the observa- 

 tions are liable; thus affording another proof of the exactness 

 of the law of periodicity above employed. Now, if we compare 

 these one with another, and with those of c as deduced by M» 

 Biot from Newton's observations, we shall have as follows : 



It appears from this comparative statement, that the forces of 

 polarisation and. double refraction in the body now examined, 

 act with much greater proportional energy on the more refrangi- 

 ble rays than in mica, sulphate of lime; and other similar bodies^ 



