Editing David Brewster (section)

===Work on optics===
Though Brewster duly finished his [[theology|theological]] studies and was licensed to preach, his other interests distracted him from the duties of his profession. In 1799 fellow-student [[Henry Brougham, 1st Baron Brougham and Vaux|Henry Brougham]] persuaded him to study the [[diffraction]] of light. The results of his investigations were communicated from time to time in papers to the ''[[Philosophical Transactions]]'' of London and other scientific journals. The fact that other scientists – notably [[Étienne-Louis Malus]] and [[Augustin Fresnel]] – were pursuing the same investigations contemporaneously in France does not invalidate Brewster's claim to independent discovery, even though in one or two cases the priority must be assigned to others.{{sfn|Chisholm|1911}} A lesser-known classmate of his, [[Thomas Dick (scientist)|Thomas Dick]], also went on to become a popular astronomical writer.

The most important subjects of his inquiries can be enumerated under the following five headings:
#The laws of [[light polarization]] by [[Reflection (physics)|reflection]] and [[refraction]], and other quantitative laws of phenomena;
#The discovery of the polarising structure induced by heat and [[pressure]];
#The discovery of crystals with two axes of double refraction, and many of the laws of their phenomena, including the connection between optical structure and crystalline forms;
#The laws of metallic reflection;
#Experiments on the absorption of light.

In this line of investigation, the prime importance belongs to the discovery of
#the connection between the refractive index and the polarizing angle;
#biaxial crystals, and
#the production of double refraction by irregular heating.

These discoveries were promptly recognised. As early as 1807 the degree of [[Legum Doctor|LL.D.]] was conferred upon Brewster by [[Marischal College]], [[Aberdeen, Scotland|Aberdeen]]; in 1815 he was elected a Fellow of the [[Royal Society|Royal Society of London]], and received the [[Copley Medal]]; and in 1816 the [[French Institute]] awarded him one-half of the prize of three thousand francs for the two most important discoveries in physical science made in Europe during the two preceding years.{{sfn|Chisholm|1911}} In 1821, he was made a foreign member of the [[Royal Swedish Academy of Sciences]], and in 1822 a Foreign Honorary Member of the [[American Academy of Arts and Sciences]].<ref name=AAAS>{{cite web|title=Book of Members, 1780–2010: Chapter B|url=http://www.amacad.org/publications/BookofMembers/ChapterB.pdf |archive-url=https://web.archive.org/web/20060618085806/http://amacad.org/publications/BookofMembers/ChapterB.pdf |archive-date=2006-06-18 |url-status=live|publisher=American Academy of Arts and Sciences|access-date=9 September 2016}}</ref>

[[File:Brewster cigar box.jpg|thumb|Inner picture of a cigar box from the early 1900s with a portrait of Brewster.]]
Among the non-scientific public, his fame spread more effectually by his invention in about 1815 of the [[kaleidoscope]], for which there was a great demand in both the United Kingdom, France, and the United States.{{sfn|Chisholm|1911}} As a reflection of this fame, Brewster's portrait was later printed in some cigar boxes. Brewster chose renowned [[achromatic lens]] developer [[Carpenter and Westley|Philip Carpenter]] as the sole manufacturer of the kaleidoscope in 1817. Although Brewster patented the kaleidoscope in 1817 (GB 4136),<ref>[http://www.ssplprints.com/image/100297/brewsters-patent-kaleidoscope-c-1817 Brewster's patent kaleidoscope, c 1817.] {{Webarchive|url=https://web.archive.org/web/20201030185833/https://www.ssplprints.com/image/100297/brewsters-patent-kaleidoscope-c-1817 |date=30 October 2020 }} ssplprints.com</ref><ref>[http://www.brewstersociety.com/brewster_patent.pdf PDF copy of the Brewster Patent GB 4136] {{webarchive|url=https://web.archive.org/web/20110721020838/http://www.brewstersociety.com/brewster_patent.pdf |date=21 July 2011 }}</ref> a copy of the prototype was shown to London opticians and copied before the patent was granted. As a consequence, the kaleidoscope became produced in large numbers, but yielded no direct financial benefits to Brewster.<ref>Gordon, p. 54</ref><ref>{{cite book|author=Myles W. Jackson|title=Spectrum of belief: Joseph von Fraunhofer and the craft of precision optics|url=https://books.google.com/books?id=VJ6SE3sbxDsC&pg=PA119|year=2000|publisher=MIT Press|isbn=978-0-262-10084-7|page=119}}</ref><ref>{{cite book|title=The Polar star, being a continuation of 'The Extractor', of entertainment and popular science|url=https://books.google.com/books?id=MpYEAAAAQAAJ&pg=PA202|year=1831|page=202}}</ref> It proved to be a massive success with two hundred thousand kaleidoscopes sold in London and Paris in just three months.<ref name="vms">[http://www.victorianmicroscopeslides.com/pdf/pcarpenter.pdf The Perfectionist Projectionist] {{webarchive|url=https://web.archive.org/web/20111007153815/http://www.victorianmicroscopeslides.com/pdf/pcarpenter.pdf |date=7 October 2011 }}, Victorian Microscope Slides. Accessed 1 August 2011</ref>

[[File:PSM V21 D055 The brewster stereoscope 1849.jpg|thumb|The Brewster stereoscope, 1849.]]
An instrument of more significance, the [[stereoscope]], which – though of much later date (1849) – along with the kaleidoscope did more than anything else to popularise his name, was not as has often been asserted the invention of Brewster. Sir [[Charles Wheatstone]] discovered its principle and applied it as early as 1838 to the construction of a cumbersome but effective instrument, in which the binocular pictures were made to combine by means of [[mirror]]s.{{sfn|Chisholm|1911}} A dogged rival of Wheatstone's, Brewster was unwilling to credit him with the invention, however, and proposed that the true author of the stereoscope was a Mr. Elliot, a "Teacher of Mathematics" from Edinburgh, who, according to Brewster, had conceived of the principles as early as 1823 and had constructed a lensless and mirrorless prototype in 1839, through which one could view drawn landscape transparencies, since photography had yet to be invented.<ref>{{cite book|last=Zone|first=Ray|title=Stereoscopic Cinema and the Origins of 3-D Film, 1838–1952|year=2007|publisher=University Press of Kentucky|location=Lexington|isbn=978-0-8131-2461-2|pages=[https://archive.org/details/stereoscopiccine0000zone/page/9 9]–10|url=https://archive.org/details/stereoscopiccine0000zone|url-access=registration|quote=mr. elliot stereoscope brewster.}}</ref> Brewster's personal contribution was the suggestion to use [[Prism (optics)|prisms]] for uniting the dissimilar pictures; and accordingly the lenticular stereoscope may fairly be said to be his invention.

A much more valuable and practical result of Brewster's optical researches was the improvement of the British [[lighthouse]] system. Although Fresnel, who had also the satisfaction of being the first to put it into operation, perfected the [[Dioptrics|dioptric]] apparatus independently, Brewster was active earlier in the field than Fresnel, describing the dioptric apparatus in 1812. Brewster pressed its adoption on those in authority at least as early as 1820, two years before Fresnel suggested it, and it was finally introduced into lighthouses mainly through Brewster's persistent efforts.{{sfn|Chisholm|1911}}