28 On the Action of the Second Surfaces of 



flowers as things of the same sort, because the same word recals 

 them ; nor is it essential that any image should be recalled to the 

 mind by the word subjects, to make it convey truths to our under- 

 standing ; as if we say, " all the subjects of God's creating power de- 

 pend on him for continued existence :" — and the figure 2 might call 

 up ment-al pictures of stars and flowers, even though the eye cursorily 

 caught it in the date of the present year, 1832. As thus — I read in 

 a book, " Pictures of 2 sorts of things, stars and flowers." Here 

 the uncommon use of the figure 2 might have so struck me as to re- 

 mind me, the next time I saw it, of the sentence in which it was thus 

 used, and the mental pictures of these objects thus be brought to my 

 mind ; and if so, is it not the same process by which the wor4 sub- 

 jects might suggest them ; and when they are suggested, does not the 

 mind, in both cases, take cognizance of them as of two sorts of things, 

 so unlike that no common name or common relation can make us 

 conceive of them as of one kind. Yet by means of a name express- 

 ing a common relation, we can reason and speculate about them in 

 connexion, though we cannot conceive of them as of the same sort. 

 If these facts are admitted, we think it must follow, that the doctrine 

 of the nominalists is no less true with regard to terms expressing ar- 

 tificial classification, than is that of the conceptualists, with regard to 

 those expressing natural classes. 



Art. V. — On the action of the second surfaces of transparent plates 

 upon light; by David Brewster, LL. D. F. R. S. Lond. &£ Edin. 



Read before the Royal Society, February 25, 1830. 



In a paper on the Polarization of Light by Reflexion, published in 

 the Philosophical Transactions for 1815, I showed that the Law of 

 the Tangents was rigorously true for the second surfaces of transpa- 

 rent bodies, provided that the sine of the angle of incidence was less 

 than tho reciprocal of the index of refraction. The action of the 

 second surfaces of plates at angles of incidence difterent from the 

 maximum polarizing angle, was studied by M. Arago, who conduct- 

 ed his experiments in the following manner. 



"With respect to this phenomenon," says M. Arago, "a remarka=- 

 ble result of experirnent may here be noticed ; that is, that in every 

 possible inclination A=A',* 



" A is the light polarized by reflexion, and A' tiiat polarized by refraction. 



