I 



190 New Properties of Light. [March, 



Article VI. 



Account of new Properties of Light. By David Brewster, LL.i>. 



F.R.S.E. 



(To Dr. Thomson.) 



DEAR SIR, 



When I received the last Number of the Annals of Philosophy, 

 I was surprised to find, from the statement in your note, tliat Malus 

 had also discovered the fact that light was polarised by refraction. 

 How far his discovery extends, and to what length he has pushed 

 his expeiiments, it is impassible for me to ascertain till I see the 

 paper which you have promised to pubhsh; but I shall not consider 

 myself as seriously anticipated, unless he has discovered the law of 

 the phenomena, the polarising powers of bundles of glass plates, 

 and the other singular results, which I have described at length in 

 my paper on that subject, which is in the possession of the Royal 

 Society of London. If Malus's memoir contains results similar or 

 analogous to these, I must then consider the labour and anxiety 

 which attended that series of experiments as completely lost. 



In the prosecution of these inquiries, to which I devote every 

 leisure moment that I can command, 1 have had the good fortune 

 to obtain a number of new results, not inferior, either in singularity 

 or importance, to any of those which have yet been published 

 respecting the affections of light. As some time must elapse before 

 the papers which I have written on these subjects can be published, 

 I have taken the liberty of transmitting to you a humed and im- 

 perfect abstract of the leading pheiwmena, and shall esteem it a 

 particular favour if you can find room for them in your Jour- 

 nal. From this abstract I have excluded the results which you 

 have already given in your history of science for 1813. The expe- 

 riments have been often repeated by myself, under vaiious modifi- 

 cations : some of them have been exhibited before the Royal Society 

 of Edinburgh, and the greater part of them have been performed 

 before some of its most disthiguished members. 



I. Experiments on the Depolarisation of Light. 



1 . Almost all minerals possess the property of depolarising light, 

 or of robbing it of its polarity, and have two depolarising axes, and 

 also two neutral axes in which no depolarisation takes place. The 

 neutral axes of mica and topaz coincide with the diagonals of their 

 primitive rhoraboidal base. Oat of 14 diamonds 7 had no neutral 

 axes, but depolarised light in every direction ; three did not depo- 

 larise it at all, and the rest produced a partial depolarisation. 



2. Caoutchouc cut in any direction, camphor, gum arahic, horn, 

 tortoise shell, &c, have no neutral axes, but depolarise light in 

 every direction. 



