CHAP. V., 1.] 



OPTICS. YOUNG. 



Many of the figures, though of hackneyed subjects, are 

 represented in a novel manner ; there seems not a 

 line drawn at random. Such figures often illustrate 

 better than pages of description, the clearness with 

 which an author has conceived to himself the neces- 

 sary results of his own principles. An example of 

 this may be found in Plate XXX., fig. 442 of the 

 first volume, which, as Arago relates, served to de- 

 monstrate to him, when he visited Dr Young in 

 1816, that he and Fresnel had been anticipated on 

 one point more than they believed. 1 



The absence of algebraic formulae from this work 

 was as characteristic of Dr Young as their copious 

 introduction into articles which he subsequently con- 

 tributed to the Quarterly Review. He had decided 

 upon writing a book without symbols, and he wrote 

 it, though it gave additional trouble both to himself 

 and the reader. 



We shall now proceed to trace the progress of the 

 undulatory theory of light, the greatest physico- 

 mathematical discovery of our time, in the establish- 

 ment of which Young acted the leading part. 



Undulations of Light Hooke and Huygens. 

 The idea of accounting for the effects and modi- 

 fications of luminous impressions by disturbances 

 propagated through a very elastic medium was by 

 no means new at the commencement of the present 

 century. We do not, indeed, attach much impor- 

 tance to the so-called anticipations of Grimaldi and 

 Hooke in the seventeenth century. The former, 

 amongst his valuable experiments on the deflection of 

 light and fringes of shadows, had used an expression 

 as to illumination being diminished by the addition of 

 light, which is true in fact, and is a correct deduction 

 from the law of interference as we now understand it. 

 Hooke, in his Micrographia, asserts light to consist 

 in " quick, short, vibrating motion ;" but his expla- 

 nation of refraction by it is altogether erroneous ; 

 and his application of it to the doctrine of the colours 

 of thin plates, though admitted by the candid Young 

 to be an anticipation (unknown to him at the time) 

 of his own, has in it no more than a germ of truth 

 (like so many of Hooke's ingenious hints, afterwards 

 claimed by him as discoveries), which yet only ex- 

 plains the fact on which it is founded by means of 

 an additional and gratuitous assumption. The germ 

 of truth in Hooke's writings is this, that the colours 

 in question depend upon a mixture of the light re- 

 flected at the first surface of the thin plate with " a 

 kind of fainter ray " propagated from the second sur- 

 face backwards : the gratuitous assumption is, that 

 " this compound or duplicated pulse does produce on 

 the retina the sensation of a yellow ;" why it does so 



is not explained. This was in 1 6 64, before even New- 

 ton was acquainted with the analysis of white light. 

 Hooke's idea that yellow, or any other colour, was 

 the result of the conflict of pulses simultaneously 

 reaching the eye, was an assertion, admissible, per- 

 haps, at that time, as expressing a fact ; but surely 

 not a proof of interference producing reinforcement 

 or annihilation of light, as taught by Young. I am 

 not aware that Hooke ever even reiterated his opinions 

 on this subject after Newton had analysed the phe- 

 nomena experimentally, and shown that the colours 

 of thin plates result from the superposition of bright 

 and dark rings of different prismatic hues, each with 

 its appropriate diameter. It was then apparent that 

 colour was only an indirect effect of interference. 



But whatever may be thought of the theories of (453.) 

 Hooke, those of Huygens deserve a far more eminent Huygens' 

 place in history. Having already been succinctly ad- ?""*' ~ de la 



3 ' -r < TM n i -i Jjumiere. 



verted to in rrotessor Playfair s dissertation, we will 

 only observe that the Traiti de la Lumiere (1690) is 

 an admirably composed and reasoned treatise on the 

 phenomena of light on the undulatory hypothesis. 

 The uniformity of its propagation through the celes- 

 tial spaces, its rectilinear course in ordinary circum- 

 stances, the laws of its reflection and refraction, are 

 there explained with a degree of elegance and preci- 

 sion which ought to have excited (we must think) 

 general attention and assent, but for the ascendancy 

 of Newton's authority, and the astonishing and beau- 

 tiful nature of the experiments on which his theories 

 were based ; whereas Huygens referred to few expe- 

 riments except those of the simplest kind, and the 

 phenomena of colour were (for good reasons) left 

 chiefly out of view. 2 Such being the case, Huygens 

 may fairly be considered as the author of the undu- 

 latory theory, which he supported by such convincing 

 proofs. 



The fundamental principle of the Huygenian (454.) 

 doctrine was the same as that which Hooke ad- Explana- 

 mitted, which was probably far older than his time,*)^ pieand 

 namely, that all space, including the interior of double re- 

 transparent bodies, contains an ether whose pulsa- fraction, 

 tions communicate the sense of light to the eye, as 

 waves in air convey to the ear impressions of sound. 

 To this he added the assumption, that in refracting 

 media, such as glass, the pulsations are retarded ; 

 whereas in Newton's theory, as is well known, the 

 propagation of light is assumed to be fastest in dense 

 media. The " law of the sines" in refraction, is de- 

 duced as a consequence ; and one of the prettiest ap- 

 plications made of it is to the phenomena of atmo- 

 spherical refraction. But the most important features 

 of the whole investigation are these two (1), the 



1 The reference of Arago, in his original eloge of Young, is to p. 387 of the Lectures, obviously by mistake. In the first 

 volume of his (Arago's) collected works it is corrected into p. 787, which is as certainly correct. I have supposed fig. 442 to be 

 more probably the one referred to than 445, to which Dr Peacock refers (Life of Young, p. 389). 



2 Nevertheless the Huygenian Theory of Light was propounded as the subject of a thesis at St Andrews by David Gregory, 

 whilst professor of mathematics there, within about a year of its publication (including also the Newtonian doctrine of gravity), 

 a pleasing proof of the activity which then reigned in that university. Principal Lee possesses the original programme. 



