(463.) 
Length of 
the waves 
of coloured 
light. 
¥ 
898 MATHEMATICAL AND PHYSICAL SCIENCE. 
which had in the first instance passed clear of the 
mirror,! Here evidently we have the required con- 
dition of double illumination with difference of paths. 
The same effect is obtained without either Inflexion 
or Reflection by refraction through an excessively 
flat prism. 
It is an easy matter (comparatively) to assign the 
lengths of a wave of light, from the intervals of the 
interference lines, or still better from the elongations 
of the coloured images produced by striated surfaces, 
the intervals of the stria 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 : 
PUKE REC Uy snascncs ces onscechanscacass és aceseess ON GOeO 
Boundary Red and Orange..........s..ssseeseeee 40,720 
ate Orange and Yellow..........s0...ss008+ 42,510 
Yellow and Green...........s.seceeees 45,600 
Groen and Blue... .0...c00isesesessssers 49,320 
Blue and Indigo...........0ss+eeseeeree 52,910 
ae Indigo and Violet............+:.0e2+00: 55,240 
BExtrOms “Violate ics). ck cc foe site eet eee ed vaece tes 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 
(Diss. VI. 
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 
in several points of view that the phenomenon of the T 
rainbow, which gave the first suspicion of the vary- 
ing refrangibility of light, and which, when explained plete with- 
and reduced to calculation by Newton, so convin- out the 
cingly proved the truth of the doctrine of the compo- 
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 
two successive impulses striking the eye and pro- bows Newton accounted for. But the spurious or Spurious 
ducing the effect of colour, is the time that an impulse supernumerary bows occasionally seen within the ®°WS 
Number of takes to travel over the length of a wave. 
vibrations 
in asecond, 
(464.) 
Opposition 
to Young’s 
views. 
tosee how almost infinitely short this must be: 460 
millions of 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. 
It might be supposed that Young’s discovery and 
its application excited the notice and applause of all 
persons 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- 
Itiseasy 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—yirst, 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. 
——_ a? 
