132 PHYSICS. 
meeting at a very acute angle do not necessarily produce an increased 
intensity of illumination, but may sometimes cause total darkness by their 
coincidence. An experiment of Fresnel’s illustrates this very satisfactorily. 
Two metallic mirrors, mc, m'c (pl. 21, fig. 68), are so placed that their 
planes are vertical, and form a very obtuse angle with each other. Let f 
be a luminous point sending rays to both mirrors and giving rise to the 
images, p, p’, lying tolerably near to each other. At a certain distance from 
the mirror the reflected rays meet each other, and form alternately light and 
dark stripes. If, for instance, the point 6 lie at an equal distance from 
p and p’, there will be a bright stripe at 6, dark ones on each side at 
s and s', bright ones again at b’ and b’”’, and dark ones at s” and s’”, &c. 
Instead of the metallic mirrors, two equal oblong plates of polished glass 
may be laid on a block of wood, touching each other along one edge. They 
should rest at each end on pieces of soft wax, so that by pressing them down 
when they are in contact, the planes of the two may be made to assume a 
very obtuse angle with each other. Fig. 88, pl. 21, exhibits a view of this 
arrangement. 
It will be necessary to explain more fully the principles of the undulatory 
theory, before these and the other phenomena of interference can be clearly 
understood. If a ray pass from A to B (fig. 89), all particles of ether 
lying between A and B oscillate up and down in directions perpendicular 
to AB. The particle whose position in a condition of equilibrium is at b, 
oscillates constantly between b’ and 6b’. At these two points its velocity is 
zero, this increasing constantly as the particle approaches the position of 
equilibrium, b, where it attains the maximum. The interval between two 
particles, b and c, which vibrate in the same phase, is called the wave length. 
It is to be observed, however, that c begins its first oscillation when b 
commences its second in the same direction. <A particle, f, half way 
between b and c, will always be in a phase of vibration directly opposite to 
them, attaining its maximum of deviation below AB, when these have 
reached their maximum on the opposite side. They are, in this case, said 
to be half a wave length apart. In general, then, two particles of ether, 
half a wave length apart, in the path of a ray of light, will be affected by 
equal and opposite velocities. The same applies to such as are distant 
3, 4,2 0f a wave length. The length of a wave and the deviation of an 
oscillation differ for the different colors, being greatest in the red, and least 
in the violet. 
To apply the preceding principles to the explanation of the phenomena 
of interference, suppose rays proceeding from f (pl. 21, fig. 68) to be 
reflected from the mirror, cm, as if they came from p, all the oscillations 
producing the ray gb being perpendicular to its path. Let a circle be 
drawn through b from a centre p; all points, s, 6’, s’, &c., lying in this 
circle, will be simultaneously placed in the same condition of oscillation by 
rays reflected from the mirror, cm. Our figure represents other concentric 
arcs drawn about p, the interval between two full arcs representing a whole 
wave length, and that between a full arc and a dotted one, only half a wave 
length. <A similar series of concentric ares has been described about p’. 
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