15 



law that the maximum of polarization takes place in reflection from 

 the joint surface of two media. The angle of incidence is comple- 

 ment to the angle of refraction, or the tangent of incidence (as the 

 author expresses it,) is equal to the quotient of the indices of refrac- 

 tion of the media. 



After describing in a series of propositions the various degrees in 

 which light becomes polarized by reflection or refraction at different 

 angles, and the number of reflections or refractions necessary to 

 effect complete polarization at various angles remote from that which 

 produces the maximum, the author investigates the origin of a cer- 

 tain quantity of unpolarized light which exists even at the maximum 

 polarizing angle in reflection from substances of high refractive 

 power ; and he shows it to depend on the different refrangibility of 

 differently coloured light. For when the incidence is such that the 

 mean refrangible rays are completely polarized, it is evident that the 

 incidence will not be such as to polarize completely either the red or 

 the violet rays, and consequently a beam composed of these will 

 appear as white light not polarized ; and when the polarization is 

 effected at the surface of substances of high refractive and dispersive 

 power, this portion will form a large proportion of the whole re- 

 flected light. On the contrary, any pencil of homogeneous coloured 

 light, though only once reflected, may be completely polarized, even 

 at the surfaces of the densest substances, if incident at an angle cor- 

 rectly adapted to its refrangibility. 



The author purposes, on some future occasion, to point out the 

 laws which regulate the polarization of light under various other cir- 

 cumstances not noticed in the present communication. 



On some Phenomena of Colours, exhibited by thin Plates. By John 

 Knox, Esq. Communicated by the Right Hon. Sir Joseph Banks, 

 Bart. G.C.B. P.R.S. Read April 6, 1815. [PAi/. Trans. 1815, 

 p. 161.] 



It is not surprising, says the author, that neither Sir Isaac Newton, 

 nor Dr. Herschel, nor any other writer who has followed on the same 

 subject, have given any explanation which appears to him to be sa- 

 tisfactory, since they have not been in possession of the phenomena 

 connected with this inquiry. After stating various objections which 

 he considers as conclusive against the alternate disposition to be re- 

 flected or transmitted inherent in the rays of light, and recurring at 

 certain equal intervals which are expressed under the name of fits by 

 Sir Isaac Newton, Mr. Knox proceeds to describe those new phe- 

 nomena which form the principal subject of bis paper, and which he 

 has been enabled to make by the assistance of the method of ob- 

 serving such appearances employed by Dr. Herschel. This method 

 consists in using the shadows of some opake substance held over 

 thin plates of glass, for the purpose of distinguishing from each other 

 the several effects produced by different surfaces employed at the 

 same time. If a plate of unsilvered glass be laid upon a table before 



