134 On the Laws of Crystalline 



only other writers who have treated of the subject of crystalline 

 reflexion. 



So early as the year 1819, Sir David Brewster published, 

 in the Philosophical Transactions, a Paper " On the Action of 

 Crystallized Surfaces upon Light." * In this Paper the Author 

 details a great variety of experiments on the polarizing effects 

 of Iceland spar. He gives the measures of the polarizing angles 

 in different azimuths, when the reflexion takes place in air; but 

 he does not notice the accompanying deviations, which were pro- 

 bably too small to attract his attention. In another instance, 

 however, he obtained very large deviations. He conceived the 

 idea of pushing his experiments into an extreme case, by mask- 

 ing, as it were, the ordinary reflecting action of the crystal, and 

 leaving the extraordinary energy at full liberty to display itself. 

 This was done by dropping on the reflecting surface a little oil 

 of cassia, a fluid whose refractive index is nearly equal to the 

 ordinary index of Iceland spar. When common light, incident 

 at 45, was reflected at the separating surface of the oil and the 

 spar, the reflected pencil was found to be partially, and some- 

 times completely, polarized in planes variously inclined to the 

 plane of incidence, the inclination going through all magnitudes 

 from to 180, as the crystal was turned round in azimuth. 

 This general result is no more than what theory would lead us 

 to expect, when the angle of incidence is nearly equal to one of 

 the angles of refraction ; but to institute a minute comparison 

 of theory with experiment would require troublesome calcula- 



axes equally inclined to the plane of incidence, but on opposite sides of it. "When 

 A'a - AS = 90, the conjugate incidences are equal, the ratio is a minimum, and 



T3 



the axes of the elliptic vibration are parallel and perpendicular to the plane of 

 incidence. When A'a = 90, or Mp. = 1, the value of T'S is a minimum, and equal 

 to tan x- 



The foregoing formulae differ slightly from those which I have given in No. I. 

 of the Proceedings of the Royal Irish Academy. The small quantity x', which 

 occurs in the latter, has been purposely neglected, as its presence interferes with 

 the simplicity of the expressions. 



* Phil. Trans. 1819, p. 145. 



