CHAP. V., 1.] 



OPTICS. YOUNG. 



99 



(460.) 

 Fundamen 

 tal law of 

 interfe- 

 rence. 



The partial reflection which always accompanies re- 

 fraction is strongly and justly insisted on as an ob- 

 vious consequence of the theory, while it requires a 

 separate hypothesis on Newton's. But the chief 

 weight is claimed for the evidence from the colours 

 of heated surfaces, of thin plates, and of diffracted 

 shadows, all of which the author explains by the 

 mixture of two portions of light conveying to the 

 same particle of ether at the same time either ac- 

 cordant or opposing motions, thus redoubling or 

 destroying the light. Of these the splendid iri- 

 descent colours reflected by surfaces having fine 

 equidistant lines drawn upon them, admit of the 

 most elementary and striking explanation. The 

 reflected image of a luminous point viewed in a 

 mirror thus cut up by parallel lines, consists of one 

 common reflection and numerous lateral images 

 which are coloured, and in which the angles of inci- 

 dence and reflection are not equal, thus contradicting 

 one of the axioms of common optics. Young showed 

 that the scattered waves of light recover the faculty of 

 appearing when the surface of the plate is seen under 

 such an angle that foreshortened intervals between 

 the scratches amount severally to the length of one 

 undulation or a multiple of it; for then the waves of 

 light scattered by the reflecting surface will not come 

 entire to the eye, but each will have a part systema- 

 tically suppressed by the non-reflecting space of the 

 groove, so that the remainders being nearly in one 

 phase, concur in making a general impression. This 

 experiment, therefore, literally presents us with the 

 paradox that by suppressing half the light, the re- 

 mainder is not suffered to be extinguished by it. The 

 different colours appear reflected at different angles, 

 because the obliquity must vary in order to be ac- 

 commodated to their several wave-lengths, and each 

 colour undergoes several repetitions corresponding 

 to breadths representing the successive multiples of 

 a wave-length. 



Precisely similar in their origin are the coloured 

 rays scattered by fibrous substances when held be- 

 tween the eye and a small point of light. If they be 

 numerous and all of the same diameter, such fibres 

 will suppress symmetrically portions of waves, and 

 suffer the oblique effect to be perceptible. Dr Young 

 most ingeniously applied this principle to construct 

 an Eriometer or measurer of the fineness of fibres. 

 The diffracted light of any order and colour from a 

 distant flame will be seen at an angle with the prin- 

 cipal or white image about four and a half times 

 greater when viewed through the down of the beaver, 

 than in the case of Southdown wool ; being the in- 

 verse proportion of the diameters of the fibres which 

 compose them. 



Fundamental Law of Interference. But the cri- 



tical and characteristic experiment of interference in 



its simplest shape was published two years later, in 



the Bakerian Lecture for 1803. 1 A small hole 

 being made with a needle point in a piece of paper 

 applied to a window- shutter, and a sunbeam being 

 directed upon it by means of a mirror from with- 

 out, a cone of light is thrown into a darkened room. 

 A slip of card one-thirtieth of an inch wide being 

 held in the sunbeam, its shadow was observed on 

 the opposite wall or on a moveable screen. There 

 were seen fringes of colour exterior to the shadow 

 on each side, such as Newton had described, and on 

 which Mr Brougham and others had made experi- 

 ments. But besides these, the narrower and less 

 conspicuous fringes seen in the interior of the shadow, 

 and first described by Grimaldi, were found by Young 

 to have this remarkable property, that they disap- 

 peared the moment that the light passing either edge 

 of the card was intercepted, whilst the exterior fringe 

 was not at all affected by that circumstance excepting 

 on the side where the light was stopped. 



Young at once perceived the significance of this (461.) 

 admirable fact. The existence of light within the ? lffr ? c " d 

 shadow at all, was evidently due to the bending of explained, 

 the wave round the opaque edge ; but the alternation 

 of light and dark spaces required the union of the 

 two lights from opposite edges, which, immediately 

 behind the centre of the obstacle, must have de- 

 scribed exactly equal paths, and therefore united in 

 the same phase ; but a little way either to the right 

 or left of the centre the phases were discordant, and 

 complete and effectual annihilation of the light re- 

 sulted. In fact, when the experiment is performed 

 under favourable circumstances, the result of the 

 union of the light is perfect blackness in these places, 

 but if half the light is stopped the dark spaces be- 

 come luminous ! 



This splendid paradox may also be demonstrated (462.) 

 without any bending round the edges of bodies, and Mixed 

 consequently without any inflexion in Newton's sense kg hts P ro- 



, * , J -, -, . , . , ,.. duce dark 



of the word ; and this simplifies the conditions mate- bands, 

 rially. In order to effect this, Fresnel (many years 

 after) produced interference bands by allowing light 

 emitted from a very small luminous point (an image 

 of the sun formed by a lens of short focus) to fall 

 upon two mirrors touching at the edge, and inclined 

 to one another at an angle very little less than 180. 

 By the common principles of reflection there will be 

 a space beyond the mirrors where the light reflected 

 from the respective mirrors overlaps, and except in a 

 single line within that space, the paths of the two 

 rays meeting in any point will be different. When 

 this difference amounts to a whole number of undu- 

 lations, an exalted brightness results ; when the un- 

 dulations arrive in opposite phases or the centres of 

 one set of waves concur with the ends of the other, 

 blackness results. The experiment may even be 

 made with a single mirror which a ray of light just 

 grazes, and after reflection mixes with the direct light 



Mitcell. Work*, vol. i. p. 179. 

 O 



