100 



MATHEMATICAL AND PHYSICAL SCIENCE. 



[Diss. VI. 



of coloured 

 light. 



which had in the first instance passed clear of the 

 mirror. 1 Here evidently we have the required con- 

 dition of double illumination with difference of paths. 

 The same effect is obtained without either Inflexion 

 or Keflection by refraction through an excessively 

 flat prism. 



(463 ) I* i g an easv matter (comparatively) to assign the 

 Length of lengths of a wave of light, from the intervals of the 

 the waves interference lines, or still better from the elongations 

 ^ ^ co i oure( j images produced by striated surfaces, 

 the intervals of the striae being given. Newton's 

 measures of the intervals between the lenses pro- 

 ducing coloured rays, gave Dr Young the following 

 for the number of undulations contained in an inch 

 producing each colour : 



Extreme Red 37,640 



Boundary Red and Orange 40,720 



Orange and Yellow 42,510 



Yellow and Green 45,600 



Green and Blue 49,320 



Blue and Indigo 52,910 



Indigo and Violet 55,240 



Extreme Violet 59,750 



Now the velocity of light is known, that is, the rate 

 of propagation of a disturbance in ether ; but the 

 duration of an impulse, or rather the interval between 

 two successive impulses striking the eye and pro- 

 ducing the effect of colour, is the time that an impulse 

 Number of takes to travel over the length of a wave. It is easy 

 vibrations to see how almost infinitely short this must be : 460 

 m a second. m flij ons o f millions of such impressions in a second 

 of time go to make up the sensation of redness, 735 

 millions of millions that of violet light. 

 (464.) It might be supposed that Young's discovery and 



Opposition its application excited the notice and applause of all 

 to Young's p ersons interested in optics. This was very far from 

 being the case. Though he brought it several times 

 in succession and in different forms before the Royal 

 Society of London, there is no evidence, so far as I at 

 present know, of his having then obtained a single ad- 

 herent. Davy was no optician ; Wollaston was too 

 cautious to commit himself, though probably giving 

 a tacit assent ; Cavendish was aged, and besides had 

 attended less to this subject than to most others ; Sir 

 William Herschel had only lately taken up phy- 

 sical optics, and that with reference to the qualities 

 of the spectrum least connected with Young's obser- 

 vations. At the Royal Institution Young vainly at- 

 tempted, in the elaborate course of lectures which he 

 there delivered for two years (1801-3) on natural 

 philosophy and the arts, to arouse a popular inte- 

 rest in the unveiling of these mysteries. The ab- 

 struseness of his discourses scared that mixed audi- 

 ence, and his colleague Davy, in a letter, incidentally 

 observes that Young would be satisfied if any one 

 would even offer criticisms on his opinions. Criticism 

 of a certain kind, however, he soon got in abundance. 

 The Edinburgh Review, in its second number, under- 



took to crush at once the theory of Young and his 

 reputation as a philosopher, and this (in singular 

 contrast to its avowed principles), not by argument, 

 but by an appeal to the weight of prescriptive autho- 

 rity in favour of the Newtonian hypothesis, conclud- 

 ing with an admonition to the Royal Society to cling 

 to its old standards and old celebrities, and to save 

 its Transactions from degenerating into volumes of 

 miscellanies. This attack, paltry as it was, seriously 

 prejudiced the reception, or even the dispassionate 

 consideration of Young's views. His anxious vin- 

 dication put forth in a separate pamphlet was unread, 

 and the doctrine of interference was first understood 

 and relished in France ten years later. 



Theory of the Rainbow. It is a matter of interest (465.) 



in several points of view that the phenomenon of the Theory of 



. , i i , i r- n ,i the ram- 



rambow, which gave the first suspicion of the vary- bow i ncon 



ing refrangibility of light, and which, when explained plete witl 

 and reduced to calculation by Newton, so convin- out the 

 cingly proved the truth of the doctrine of the compo- interfe- 6 

 site nature of white light, was destined in the hands rence. 

 of Young and of his successors to yield one of the most 

 refined evidences of the extensive application of the 

 doctrine of interference. The general fact of the ar- 

 rangement of colour in the primary and secondary 

 bows Newton accounted for. But the spurious or Spurious 

 supernumerary bows occasionally seen within the " OW8< 

 primary, and far more rarely beyond the secondary, 

 consisting of reddish and greenish bands, remained 

 unexplained. The brilliancy of any given portion of 

 the rainbow depends upon the deviation of the sun's 

 rays by two refractions and one reflection, approach- 

 ing to a limit which it cannot overpass. But except at 

 this precise limit an amount of scattered light will 

 reach the eye, which, though not reflected under the 

 most favourable circumstances, yet is still sufficiently 

 intense to be visible. This light must be composed, as 

 Young showed, of two portions, entering the eye in 

 the same direction, but which have pursued different 

 paths within the drop, and which never coincide except 

 at the extreme geometrical limit before mentioned. 

 When one of these paths differs from the other by the 

 length of half a wave of the particular kind of light 

 considered, darkness will result, but a feebler maxi- 

 mum will be again attained when the interval rises 

 to a whole wave-length, or to two or more. Hence 

 these consequences follow -first, that the bright part 

 of each colour is limited by its self-destruction to a 

 narrow band, and thus the purity of prismatic colour 

 so striking in a well-formed rainbow is preserved ; 

 secondly, that each colour may attain (by interference) 

 a second and a third maximum, corresponding in fact 

 to the position of the spurious bows ; thirdly, that 

 these phenomena of perfect definition of the primary 

 and secondary bows, and of repeated maxima in the 

 supernumerary bows, depend essentially on this condi- 

 tion, that the drops of falling rain shall approach to 



1 This experiment is usually (and justly) ascribed to an eminent and amiable British philosopher. But it had already been 

 performed by Fresnel with a special object. See Ann. de Chimie et de Physique, second series, xv. 382. 



