14 SIXTH REPORT. — 1836. 



ner that the interior forces, which produce double refraction, extend 

 within the sphere of the ordinary reflecting force, and modify its ac- 

 tion not only in polarizing common light and changing the planes 

 of polarized light, but in reflecting diflferent quantities of light at 

 dift'erent angles of incidence. 



These experiments excited no attention among those who were 

 studying the theories of light till 1835, when they attracted the no- 

 tice of Mr. MaccuUagh, of Trinity College, Dublin, who was then 

 engaged in investigating the laws which regulate the reflexion and 

 refi'action of light at the separating surface of two media. 



From principles analogous to those employed by Fresnel, Mr. 

 MaccuUagh has anticipated eflTects quite the reverse of those de- 

 duced from my experiments ; and in order to account for the latter 

 he was obliged to abandon to a certain degree the physical ideas of 

 Fresnel in so far as to make the vibrations of the wave parallel to 

 its plane of polarization, in place of perpendicular to it. From the 

 theory thus modified Mr. MaccuUagh has shown that when a ray is 

 polarized by reflection from a crj'stal the plane of polarization de- 

 viates from the plane of incidence, except when the axis lies in the lat- 

 ter plane. The formula which expresses this deviation represents 

 \'ery accurately the measures of the polarizing angles in diflFerent 

 azimuths, which I have obtained in the only surface in which the 

 exception is true ; but at all other inclinations of the reflecting plane 

 to the axis, the formula and the theory are in fault, as there is a 

 large deviation when the axis or principal section of the crystal is in 

 the plane of reflexion. 



After the publication of my paper of 1819 I had more than once 

 resumed the subject ; but the difficulty of obtaining highlj'^ polished 

 surfaces of calcareous spar at diff^erent inclinations to the axis forced 

 me to abandon the inquiry. When I found, however, that Mr. 

 MaccuUagh had succeeded in deducing from theory the general fact 

 of a deviation increasing as the refractive power of the medium ap- 

 proached to that of the spar, I had no doubt that he would bring the 

 more complex phenomena under the dominion of theory, provided I 

 could furnish him with their physical law. In this expectation I 

 devoted my whole time to the inquiry during the last winter, with 

 more knowledge of the subject and better means of observation ; and 

 I should have made much greater progress than I have done had I 

 been able to procure crystals of calcareous spar suited to my pur- 

 pose. In this difficulty I applied to the British Museum through 

 Mr. Konig, for some useless fragments of their specimens, but I 

 was mortified to find that an Act of Parliament prohibited even the 

 dust of a ciystal from being removed from its walk. 



The difficulty which I experienced in obtaining crystals with 

 planes sufficiently regular and polished, obliged me to work with 

 artificially polished surfaces ; and I have to express my obligations 

 to Mr. Nicol, of Edinburgh, for the kindness and the love of science 

 which led him to polish with his own hands the surfaces which I 

 required. 



