Optical 
paradox. 
(459.) 
The Erio- 
meter. 
(460.) 
Cuar. V., § 1.] 
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 pointof 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- 
Fundamen- tical and characteristic experiment of interference in 
tal law of 
interfe- 
rence. 
its simplest shape was published two years later, in 
OPTICS.— YOUNG. 
897 
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 
admirable fact. The existence of light within the 
(461.) 
Diffrac- 
tion-bands 
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 
lights pro- 
duce dark 
of the word ; and this simplifies the conditions mate- hands, 
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 
1 Miscell. Works, vol. i. p. 179. 
VOL. I. 
5x 
