NEWTON, ISAAC. 



NKWTON, ISAAC. 





by Dr. Yinoeot in 1S. Its perusal gave rise to man; remark* from 

 Dr. Hooka respecting the deduction of the law of the inverse square 

 of tin.- disunw froui the seoond law of Kepler, to which he laid claim. 

 Hooke'i preUaaions upon this head were never tnade out ; and from 

 tti known mediocrity of bis mathematical attainments, as well as his 

 general disposition to appropriate every new discovery to himself, 

 they are l.ok.d upon as Yery doubtful. Newton however having 

 apparently replied to his remarks with more warmth than discretion, 

 ho (Newton) afterwards expressed a willingness to compromise the 

 dispute by the insertion of a scholium (lib. i prop. iv. cor. 6), wherein 

 the c .ntrovcrted deduction is attributed independently to Sir Christo- 

 pher Wren, Hooke, and Halley. The printing of the 'Priudpia' was 

 superintended by Dr. Halley, and the expense defrayed by the Royal 

 Society. It appeared in 1637, London, 4 to. For an account of this 

 memorable work, the reader is referred to Pm.xciriA, in ABIS AND/ 

 Sc. Div. 



The theory of universal gravitation advanced by Newton, wherein 

 each particle of matter is supposed to attract all other matter in the 

 direct proportion of its mass, aud inversely as the square of the 

 distance, is, from its nature, insusceptible of direct demonstration, 

 aud could only be established by showing that the phenomena of 

 nature were in no instance opposed to such a supposition, and that it 

 wns sufficient to the explanation of those phenomena, Newton did 

 much towards this; but the completion of the proof required the 

 labours of many succeeding mathematicians and astronomers, and this 

 may, in some measure, account for the qualified reception which the 

 theory met with at the hands of Huyghens, who, though he acknow- 

 ledged the mutual gravitation of the masses of matter, refused his 

 assent to the assumed attraction between their ultimate particles. The 

 opposition to the theory made by Leibnitz, the erroneous and unphilo- 

 sophical principles which be endeavoured to substitute in its stead, 

 and the meanness with which he sought to injure the reputation of 

 Newton by designating the theory subversive of true religion, are 

 equally prejudicial to his scientific and moral fame. The long interval 

 that elapsed before the writings of Newton began to be generally 

 understood and his doctrines appreciated, is attributable exclusively, 

 as regards England, to their inherent abatruaeness. On the Continent 

 they had further to contend with established prejudices in favour of 

 the doctrines of Descartes. 



The Utter half of the 17th century is not less remarkable for its 



optical discoveries than for the zeal with which the physical sciences 



generally were prosecuted. The proportionality of the sines of the 



angles of incidence and refraction, which furnishes an easy explanation 



of all the circumstances attending the simple refraction of homogeneous 



light, had been discovered, and the discovery attributed to Descartes, 



though now known, on the authority of Vossius and Huyghens, to 



have been due to Knell, a Dutch physician, who died in 1626. From 



this law Descartes hail been able to explain the theory of the refracting 



telescope, but had fallen into error by attributing the defects of such 



instruments solely to the spherical aberration of the lenses employed 



in their construction. Ha<i such been the case, it is obvious that the 



whole of the aberration might have been removed by merely varying 



the form of the lenticular surface. Descartes accordingly, in his 



'Dioptric*, 1 published in 1629, and James Gregory, in his 'Optica 



Promota,' published in 1663, hod investigated the forms which they 



believed it would be uecratary aud sufficient to give to the lenses, in 



order that parallel rays transmitted through them should converge to 



a point or focus with mathematical accuracy. The aberration ol 



sphericity was however but a trifling impediment to the perfection 



of dioptrical instruments, compared with what is now designated 



chromatic aberration, or the aberration of refrangibility, which arises 



from the then undiscovered fact that light is not a homogeneous 



substance, but composed of rays, some of which are more susceptible 



of refraction than others. In the year 1666 we learn (' Letter ol 



Newt<iu to Oldenburg,' ' 1'hiL Trans.') that Newton, in common with 



many other philosophers at that time, occupied himself in the attempt 



to grind object-glasses in conformity with the precepts of Descartes 



and Gregory, aud while thus occupied h j seenii to have conjectured 



that the defects of refracting telescopes uiuht arise from some other 



cause th.-in that which bad hitherto been exclusively assigned. Either 



this conjecture or accident led him to consider the phenomena of the 



prUmatic s|iectruui, which was thm well known to philosophers, 



although it bad been explained by none. In doing so he remarket 



that the angle subtended at the aperture of his shutter by the length 



of the spectrum was cotuidi-ntbly greater than that subtended by the 



tun's diameter, though, according to the received law of refraction 



these angks ought to have been sensibly equal. After modifying the 



experiment in various ways, and assuring himself that this discrepance 



did not arise from any irregularity in his prism, the thought fortunately 



suggested itself of trying Uie wparate rflVct of the pri-m upon each o 



the coloured lights of the pectrum. Accordingly, having tranamittet 



them successively through a second priim at the same angle of incidence 



he found 1, that the colour was in no degree affected by this seconi 



refraction ; 2, thai the spectrum formed by each colour occupied a 



distinct position on the screen, so that the deviation from the direction 



of the primitive ray was different in all, being greatest in the viole 



and least in the red. He therefore came to the important conclusion 



"that light was not homogeneous, but composed of rays, some o 



hich were more refrangible than others." This discovery must hare 

 >een made subsequent to the publication nf Wallia'a optical lectures, 

 which appeared in 1669, for otherwise it is improbable that Newton, 

 <o whom the revisal of the manuscript was confided, aud who receives 

 he author's acknowledgments for having " corrected several over- 

 sights, and made some important corrections," would have permitted 

 his friend to promulgate views so erroneous. 



In the above year however Newton began to deliver a course of 

 ecturea on optics at Cambridge, and the composition of white light 

 brmed part of the course. Seeking the complete development of the 

 consequences of his discovery, " he was conducted," says M. Biot, " to 

 a multitude of observations no less admirable for their novelty and 

 mportance, than for the sagacity, address, aud method with which he 

 contrived, executed, and linked them one with the other. He thence 

 composed his system of optics, wherein the fundamental proper' 

 ight were for the first time unveiled and established, and classed 

 according to pure experience alone, without the slightest intermixture 

 of hypothesis ; a method of procedure which won then as surprising 

 and as little heard of as the properties themselves : . . . both the 

 ihysical and natural sciences were at this time so mixed up with mere 

 ipiuions, that few persons were capable of discriminating between a 

 vogue conjecture and a precise idea, between a physical hypothesis and 

 a law of nature rigourously demonstrated." 



Although Newton had been thus successful ill his optical inquiries, 

 and had detected the principal source of indistinctness in refracting 

 telescopes, he not only did not see how this indistinctness rnijit be 

 removed, but even designated any attempt as 'desperate' (' Optics,' 

 ib. L prop. 7) ; and the long interval which elapsed before the discovery 

 of the achromatic telescope may be attributed to the unmerited reliance 

 which in this instance was placed in his decision. He regarded as a 

 self-evident truth, which it therefore needed neither reason nor obser- 

 vation to confirm, that the spectra formed by different refracting 

 media were always of equal length whenever the refraction of the 

 mean ray was the same ; in other words, that the dispersive powers of 

 different media were the same where their indices of refraction corres- 

 ponding to the mean ray were equal ; and although the fact was 

 controverted by some of the more respectable continental philosophers 

 who assailed his doctrines, the circumstance appears to have neither 

 shaken his belief, nor to have suggested the propriety of convincing 

 liis opponents by an appeal to experiment. An apparent analogy 

 also between the lengths of the colours of the sprctrum ami the 

 divisions of the harmonic chord, led him to suppose that the refractive 

 indices of any medium corresponding to the different coloured rays 

 might be deduced from a knowledge of the index of refraction corres- 

 ponding to the mean. Had he happened to have repeated his experi- 

 ments with different refracting substances, he could scarcely have 

 failed to recognise the difference of their dispersive powers even when 

 the refraction of the mean ray is the same. Many of his experiments 

 he did repeat with a prism of water, but in this instance ho mixed 

 with the water "a little sugar of lead " (Brewster), his object bi-iuj; 

 merely to increase the refractive power of the water ; but he thereby 

 unknowingly also increased its dispersive power, and probably rendered 

 it sensibly equal to the dispersion of the gloss prism previously em- 

 ployed. By these mistakes he was induced to forego all attempts to 

 perfect the refracting telescope, and directed his attention to the 

 construction of reflecting telescopes on the principles pointed out by 

 James Gregory, in his ' Optica Promota,' 



In 1668-69 he bod succeeded; and in 1671 he executed a second 

 telescope " with his own hands," which is still carefully preserved in 

 the library of the lioyol Society. It was in order to explain the 

 phenomena exhibited in investigating the colours of thin transparent 

 substances, and the partial reflexion and transmission of light incident 

 upon diaphanous surfaces, that Newton devised his theory of " fit* of 

 easy reflexion and transmission." [Liaai, in ARTS AND 80. Div.] 

 Speaking of Newton's theory of colour, Sir John Herschel remarks, 

 " it is a theory of extraordinary boldness and subtlety, in which great 

 difficulties are evaded by elegant refinement*, and the appeal to our 

 ignorance on some points is so dexterously backed by the weight of 

 our knowledge on others, as to silence if not refute objections which 

 at firnt sight appear conclusive against it." ( Kncyc, Hetrop.,' art 

 ' Light.') 



In 1672 Newton was elected member of the Royal Society. In 1675 

 be received a dispensation from Charles IL to continue in his f. How- 

 chip without taking orders. About the year 1687 he was appointed 

 one of the delegates to defend the privileges of the University of 

 Cambridge against the attacks of James II. In 1633 he took his seat 

 in parliament as one of the representatives of the university, and wa,s 

 resident in Londou until the dissolution of parliament, which took 

 place the following year. He was again returned in 1703, but lost his 

 election ill 1705. On no occasion does he appear to have taken any 

 conspicuous part in the debates of the House. Prior to the year 

 1695, when be was appointed warden of the mint, with a salary of 

 from uOOi to 6001. per annum, his pecuniary circumstances are said to 

 have been rather straitened, and whatever may have been hi* 

 own expectations, his friends seem to have reckoned upon some token 

 of liberality from Charles II. and his successor. The profligate reign 

 of the former, and the opposition which Newton hod given to the 

 wishes of Jame, sufficiently account for their expectations having 



