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the discovery of the tangent law now always identified with 
his name. But there were other occasions when some 
mere mathematician stepped in to take up the elaborate 
facts which Brewster had elaborated, and from them in a 
few minutes to deduce a law for which he took the credit 
of discovery. ‘It seems to us,” writes Prof. Tait in an 
article which appeared in the Scofsman shortly after 
Brewster’s decease, “that sufficient allowance has not 
been made for the natural irritation which such treatment 
was certain to cause, especially in a high-souled and 
single-minded man incapable of treating others as he felt 
himself treated. His biographer will have a painful, but 
a necessary and salutary, task to perform in gibbeting 
such thankless parasites. Many a much-praised scientific 
article—volume even—may be found where the facts are 
taken mainly from Brewster, though his name is barely 
mentioned. He was driven by such treatment into fre- 
quent disputes about priority, and in general he was 
successful, though often before the final settlement of the 
question the obnoxious paper had found its way to a non- 
scientific public, and even to foreign journals. It is 
always a difficult matter to determine what is the proper 
course for a philosopher under such circumstances. Few 
have the calmness to rely upon the almost invariably just 
decision of posterity ; and most of those who do so go 
unrecognised to their graves.” 
In 1816 Brewster announced his discovery of the cause 
of the colours playing over the surface of mother-of- 
pearl, and of the possibility of transferring them to casts 
taken in wax, isinglass, and fusible metals. He also 
investigated the images and fringes of colour visible in 
some natural specimens of calc-spar, and turned his 
attention, though this time with only incomplete results, 
to the production of tints by multiple reflexions at the 
surfaces of polished metals. When in 1830 he returned 
to the subject, there resulted a remarkable memoir on 
Elliptical Polarisation, which appeared in the Phz/o- 
sophical Transactions. In 1817 he discovered the whole 
class of biaxial crystals, and triumphantly deduced the 
- law of their action on light, thereby solving the difficulties 
which had perplexed Biot anc Arago. He even sketched 
out a relation between the primitive forms of crystallisa- 
tion of minerals and their optical behaviour. 
His attention was next directed to the question of the 
absorption of light. In this department of science he 
made many observations. He was the first to observe in 
any systematic way the effects of absorption upon the 
prismatic spectrum. He reinvestigated the solar lines 
discovered by Wollaston and Fraunhofer, and observed 
even a larger number of them than the latter had detected. 
He made the important observation that many of these 
lines are due to absorption by the earth’s atmosphere ; 
and in one of the latest of his contributions to science—a 
joint paper by himself and Dr. J. Hall Gladstone, which 
appears in the Philosophical Transactions for 1860—he 
returned to the subject with unabated vigour and unsur- 
passed perspicacity of thought. He also discovered the 
power possessed by nitrous oxide gas to produce absorp- 
tion lines, and he noted the great and extraordinary 
increase in their number and density when the gas is 
heated. ‘‘The power of heat alone to render a gas which 
is almost colourless as red as blood, without decomposing 
it, is in itself a most singular result; and my surprise was 
greatly increased when | afterwards succeeded in rendering 
the same pale nitrous gas so absolutely black by heat, 
that not a ray of the brightest summer sun was capable 
of penetrating it.’’ Indeed he seemed to be here on the 
very verge of the discoveries on the spectroscopic signifi- 
cance of the width and frequency of the absorption lines 
which have been made by Mr. Lockyer, M. Janssen, and 
others during the present decade. In 1831 Brewster 
published his ‘‘New Analysis of Solar Light,” the new 
analysis being nothing else than the operation of looking 
at the solar spectrum through coloured absorbent media, 
NATURE 
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[Dec. 15, 1881 
It was this series of experiments which led him to con- 
ceive the theory of the three primary colours which he 
so resolutely maintained against all opponents till his 
dying day. Through his red glass he could see light 
through a considerable range of the visible spectrum, and 
therefore, he concluded, there is some red in all parts, 
but with different degrees of brightness. The yellow and 
the blue were, he held, also distributed, each with a maxi- 
mum of its own, throughout the range of the whole light. 
He believed that he had proved the conversion of blue 
rays into violet ones by viewing them through an ab- 
sorbent medium. ‘We must remember,” says Prof. 
Tait, by way of apology for the persistence with which 
Brewster clung to his pet theory, “that he trusted to an 
eyesight that had rarely deceived him—an eyesight once 
so perfect that he is one of but a very few who have seen 
the extraordinary ultra-red rays which he was the first 
to discover as visible light.” 
One of his researches connected the subject of absorp- 
tion with his work on polarisation. He investigated the pro- 
perty known as dichroism possessed by a great number of 
coloured crystals, tourmaline, Brazilian topaz, and others, 
a property which has lately given rise to several impor- 
tant investigations by physicists in Germany and in 
England. He showed how the absorbed tints are altered 
by heating, and here he anticipated a point in the elec- 
tromagnetic theory of light which was then of course 
quite undreamed of. 
To enumerate the whole of Brewster’s researches would 
occupy so many columns that only a few of the more 
prominent must now be adverted to. Optical illusions of 
sundry kinds, fluid cavities in crystals, polarisation of the 
sky, phosphorescence, fluorescence, photography, the 
optical properties of agate, opal, and labradorite, the 
magic mirror of Japan, and the theory of binocular 
vision, all claimed their notice and formed the bases of 
many careful researches. The experimental researches 
of Brewster in optics are in fact paralleled only by those 
of Brewster’s great contemporary Faraday in electricity. 
Brewster was the inventor of several well-known optical 
instruments. The 4a/eidoscope, which was brought out 
in 1816, created such a furore that 30,000 were sold 
in a few days. His monochromatic lamp appeared in 
1823. In 1849-50 he brought out his lenticular s/ereo- 
scope (an improvement upon Wheatstone’s reflecting 
stereoscope of 1838), and a binocular camera, for use in 
producing stereoscopic pictures. Still more important, 
though far less widely known, was his discovery of the 
application of lenses and combinations of lenses to light- 
houses. This was in 1812; in 1820 he was urging the 
adoption of his system on those in authority—two years 
before Fresnel, who usually gets the credit of this | 
application, had begun his work. 
His objections to the undulatory theory of light endured 
to the last, when he stood almost alone in his refusing 
explicit adherence to the theory. Trained himself in 
another school of thought, and accustomed through long 
years to the Newtonian theory, it is not remarkable that 
in the absence of mathematical predilections the mathe- 
matical intricacies of the fabric woven by Fresnel had 
little charm for him. And if we find it hard to realise the 
slowness of minds like Brewster’s to receive the undu- 
latory theory as an established truth, we may perhaps 
find no inapt parallelism in the repugnance felt even 
amongst some of the “crowned heads of science’’ at the 
present day towards entertaining the still more modern 
electromagnetic theory of light in which the undulatory 
theory is fast being swallowed up piecemeal. 
Brewster’s literary activity was simply extraordinary. 
He brought out the “ Edinburgh Encyclopedia ” between 
the years 1808 and 1830, writing many of the articles 
himself. To the seventh and eighth editions of the 
“Encyclopedia Britannica” he contributed the articles 
on Electricity, Magnetism, Microscope, Optics, Stereo- 
