CHAP.V.,!.] 



OPTICS. YOUNG. 



of his election as a foreign member of the Koyal 

 Society of London. It is to be regretted that Savart 

 never published a connected account of his obser- 

 vations. He had caused to be collected at the College 

 de France, where he was professor, an unequalled col- 

 lection of acoustical apparatus, a great deal of which 

 was contrived by himself, and where he delivered 

 extensive courses of lectures on this subject alone. 



Several English philosophers, in particular Pro- (443.) 

 fessor Willis, Mr Hopkins, and Mr Wheatstone, have Othei \ 

 written several important detached memoirs on par- expei-i-^ 

 ticular practical points, for which we must refer to ments. 

 special treatises. 1 The first and last of these gentle- 

 men, together with some Germans, have approxi- 

 mated in some degree to the formation (on empirical 

 principles) of a speaking machine. 



CHAPTER V. 



jptics in 

 the 18th 

 3entury. 



(445.) 

 Thomas 

 Young. 



OPTICS. 



1. THOMAS YOUNG. The Undulatory Theory of Light. Its history from the time of Hooke 

 and Huygens. The Law of Interference. Its application to Diffraction to the Rainbow 

 and to other subjects. The Theory of Polarization referred to another section. 



with a man altogether beyond the common standard, 

 one in whom natural endowment and sedulous cul- 

 tivation rivalled each other in the production of a 

 true philosopher ; nor do we hesitate to state our 

 belief that since Newton, THOMAS YOUNG stands un- 

 rivalled in the annals of British science. 



He was born at Milverton in Somersetshire on the (445.) 

 13th June 1773, and his biographers dwell with com- His early 

 placency on the prodigies of his youth, uncertain as ed " catlo p 

 such attainments confessedly are in stamping the men t s . 

 greatness of the future character. At the age of 

 fourteen he had learned (principally for amusement) 

 seven languages besides his own, and besides had 

 made a point of mastering every subject, whether in 

 science or miscellaneous knowledge, which he had 

 once determined upon prosecuting. Thus, whilst 

 studying botany he resolved to learn how to make a 

 microscope, but finding in Martin's Optics the nota- 

 tion of fluxions, he became his own preceptor in that 

 branch of analysis. " He acquired a great facility 

 in writing Latin. He composed Greek verses which 

 stood the test of the criticism of the first scholars of 

 the day, and read a good deal of the higher mathe- 

 matics. His amusements were the studies of botany 

 and zoology, and to entomology, in particular, he at 

 that time paid great attention." 3 Dr Young's edu- 

 cation was almost completely private. Having been 

 brought up according to the tenets of the Society of 

 Friends, he had not thought of going to Cambridge 



(444.) E history of Optics in the eighteenth century is 



Small pro- one of the blankest pages of scientific story ; at least 

 stress of if we allow Bradley's discovery of aberration to be (as 

 it really is) rather an astronomical than an optical 

 discovery. The most notable advance was unques- 

 tionably the invention of the achromatic telescope 

 as narrated in the Fifth Dissertation, 2 founded on 

 the proof of Newton's oversight in the matter of dis- 

 persion. The construction of refracting telescopes 

 made rapid advancement in the workshop of Dollond, 

 whilst reflecting telescopes, in the hands first of Short, 

 but far more conspicuously, of Sir William Herschel, 

 were shown to be capable of making unimagined dis- 

 coveries. The geometrical theory of optical instru- 

 ments was also greatly improved ; but all this led 

 to little increased knowledge concerning Light itself. 

 If we except the valuable though imperfect treatises 

 of Bouguer and Lambert on the subject of photo- 

 metry, and a paper by Mr (now Lord) Brougham in 

 the last years of the century, recalling attention to 

 the inflexion of light, the history of Physical Optics 

 (as that part of the science touching more imme- 

 diately the nature and qualities of light is now usu- 

 ally termed) is almost a blank from the publication 

 of the Optics of Newton in 1704 to that of Young's 

 papers almost one hundred years later. 



It is not therefore from overlooking Young's pre- 

 decessors that we open our review of the recent pro- 

 gress of optics with his discoveries. We here meet 



1 See also Herschel on Sound (Encyc. Metrop.}; and Whewell's History of the Inductive Sciences, vol. ii. 



2 It may be mentioned, however, that the credit usually ascribed to Dollond must be divided, at least, with Mr Hall, a pri- 

 vate gentleman of Worcestershire, who not only imagined but constructed achromatic telescopes as early as 1733 (Gentleman's 

 Magazine, 1790, and Phil. Mag., vol. ii.) The improvement by Dr Blair of Edinburgh has been alluded to in Sir John Leslie's 

 Dissertation. It consisted in enclosing fluids in the object glass, of such composition as to disperse the several rays of the spec- 

 trum in the tame proportion to one another (though not to the same absolute amount) as the glass with which it was combined ; 

 thus rendering the achromatism more perfect. 



3 Memoir of the Life of Thomas Young, M.D. 8vo, 1831. The present section of the Dissertation was written shortly be- 

 fore the publication of the Life and Miscellaneous Works of Dr Thomas Young, for which the public is indebted to Dr Peacock, 

 Dean of Ely, the possession of which would have materially facilitated my task. Wherever Dr Peacock's information has 



