Ixxi 



nation every mineral that came in his way. Frequently one of these sub- 

 stances would form the subject of a monograph, as diamond, or amber, the 

 double cyanide of platinum and magnesium, the felspar of Labrador with 

 its changeable tints, or Glauberite with its one axis of double refraction for 

 the violet, and two axes for the red ray. The prismatic spectrum arrested 

 his attention, and in 1834 he announced the absorption of certain rays by 

 the earth's atmosphere, and by nitrous gas ; while eight years afterwards 

 he pointed out the existence of luminous lines in certain flames correspond- 

 ing to those defective in the light of the sun ; but he missed the beautiful 

 explanation of Kirchhoff. He also investigated the phenomena of diffrac- 

 tion and dichroism, and of late years exhibited to the British Association 

 the tints of a soap-bubble, or of decomposing glass rendered still more lively 

 by being viewed through a microscope. Indeed his last legacy to science 

 was a paper on Film forms. 



The best monument to his fame is perhaps his investigation of polarized 

 light. Malus had first set foot on this domain, but his premature death 

 left it open to the entrance of Brewster, and what wonderful regions did he 

 explore ! It not unfrequently happened that some other philosopher, with 

 perhaps a profounder knowledge of mathematics, stepped in and deduced 

 important laws ; but sometimes he himself arrived at the higher generali- 

 zations ; as, for instance, may be cited that of the refractive index of a sub- 

 stance being the tangent of its polarizing angle. But he was not always 

 fortunate in his theories ; thus his ingenious view of solar light, as composed 

 of three primary colours (red, yellow, and blue) forming coincident spectra 

 of equal length, has been shown to be completely fallacious. Yet he never 

 abandoned his theory ; a fact which we are disposed to attribute, not to a 

 want of conscientious truthfulness, but rather to an inability to appreciate 

 the real bearing of an argument, and to an over confidence in his own 

 memory and the testimony of his senses. 



During his optical investigations Sir David often turned from the phe- 

 nomena seen to the organ of sight, and experimented on that wonderful eye 

 which saw bands in the red rays less refrangible than Fraunhofer's A. Of 

 late years especially he examined the functions of the retina, the foramen 

 centrale, and the choroid coat of the eye of animals ; he wrote several 

 papers on the musccs volitantes, and explained many peculiarities of single 

 and binocular vision, and not a few optical illusions. 



"While pursuing these researches on light, he made frequent excursions 

 into other regions of science ; he discovered fluids in the cavities of some 

 of the minerals he was examining, and these must be investigated ; he 

 wrote much on the mean temperature of the globe ; his attention was 

 attracted at one time to fossil bones from Ava, at another to the varnish- 

 trees of India ; while systems of double stars, and the pyro-electricity of 

 minerals shared the notice of his comprehensive mind. 



As an inventor of new apparatus Brewster also acquired no little renown. 

 His first paper on this subject appears to have been " Some remarks on 



