ANNALS 



OV 



PHILOSOPHY. 



MARCH, 182L 



Article I. 



On the Action of Crystallized Bodies on Homogeneous Light , and 

 on the Causes of the Deviation from Newton's Scale in the 

 Tints which many of them develope on Exposure to a polarised 

 Ray. By J. F. W. Herschel, Esq. FRS. Lond. and Edin. 

 (With a Plate.) 



{Concluded from 1^. 132.) 



Our equation {h) gives room for a remark of some conse- 

 quence, as it affords a striking verification of the theory here 

 dehvered. It will be observed that this equation does not 

 involve t, and in consequence, the angle 6 determined from it, at 

 which the coincidence takes place, is the same for all values of 

 t, or for all thicknesses of the plate. The observations of the 

 tints in the tables given above afford us ample means of putting 

 this remarkable consequence to the test of experiment. In fact, 

 in the three series of tints observed in sulphate of baryta, the 

 apparent angles between the axes for the mean red rays are 

 respectively 62° 0', 62° 2\ and 61° 53', the mean of which is 

 61° 58', while the apparent angles between the virtual poles in 

 the same series are 72° 46', 72° 36', and 72° 47'. The semi- 

 excesses of the latter angles over the mean value of the former 

 are the apparent angular distances of the virtual poles from the 

 axes of mean red rays, and are respectively 5° 24', 5° 19', 5° 25', 

 neither of which differs more than 4' from the mean. 



To ascertain the real value of 5, we have only to compute the 

 angles of refraction. In the specimen employed, I found 1*6475 

 for the index of ordinary refraction, and the angles of incidence 



Ntw Series, yoL» I, , l 



