234 SIR DAVID BREWSTER ON THE 



In this way I found that the ordinary reflecting force of the spar was nearly 

 reduced to nothing, and was almost entirely under the dominion of the force 

 which emanated from the crystal. Light incident on the crystalline surface was 

 no longer polarised in the plane of reflexion, but in planes inclined to the prin- 

 cipal section of the crystal, the rotation or deviation of the plane increasing with 

 the angle which the plane of reflexion formed with the principal section, and was 

 so related to the angle which the incident ray formed with the axis of the crystal, 

 that the Sine of half the rotation, or deviation, was equal to the square root of the 

 Sine of the incident ray to the axis. 



The bearing of these results, as published in the memoir already referred to, 

 upon the theory of Light, directed the attention of mathematicians to this subject, 

 and I Avas thus induced to resume the inquiry, by investigating the action of sur- 

 faces variously inclined to the axis of calcareous spar, — to study the effect of fluids 

 of different refractive powers, in reducing the action of the reflecting force, 

 and to ascertain the influence of the surfaces thus modified upon light polarised in 

 planes differently inclined to the principal section of the crystal. 



Some of the results thus obtained were communicated at different times to 

 the British Association, and were found by the late Professor Maccullagh of 

 Trinity College, Dublin, to be deducible from the Undulatory Theory ; but other 

 results, in which the phenomena were asymetrical with respect to the principal 

 section of the crystal were less accordant with theory. Professor Maccullagh was 

 therefore desirous to observe the phenomena himself; and having resolved to 

 have an apparatus constructed more complex and perfect than the one I used, I 

 willingly left the subject in the hands of my distinguished friend. What expe- 

 riments he made, or whether he made any, before the sad and sudden close of his 

 life in 1847, I have not learned. The subject has therefore again come into my 

 hands ; and having been encouraged by Professor Stokes of Cambridge to publish 

 my experiments, as having an important bearing on the theory of light, I now 

 submit them, incomplete and imperfect as they are, to the consideration of the 

 Society. 



The experiments which I published in 1819 were made on the cleavage 

 planes of the primitive rhomb of calcareous spar, but those which I am about to 

 describe were made on artificial faces, carefully prepared for me by the late 

 Mr William Nicol, the ingenious inventor of the polarising prism which bears his 

 name. 



I could have wished to repeat some of these experiments with a better appa- 

 ratus, and with freshly polished surfaces of calcareous spar, but the sharp vision 

 and the sensitive retina of early or middle life are necessary for the observation 

 of delicate and almost evanescent phenomena. 



In the following Table I have given the observed polarising angles of the sur- 

 faces employed, and of their inclination to the axis of the rhomb :— 



