134 PHYSICS. 
parallel rays, and mz the position of the incident plane wave. At the 
moment that this wave reaches n, a system of spherical waves will be 
diffused in the two contiguous media, which, however, will be propagated 
with unequal velocity in the two media, owing to the different elasticity of 
the ether contained in them. Supposing the second medium to be more 
refractive than the first, then the wave propagated in it from m, will reach 
the surface of a sphere, whose radius, mo, is less than nk, in the same time 
that the plane wave occupies in traversing the space nk. Moreover, the 
plane wave reaches m' and n’' simultaneously, and passes from x‘ to k, while 
the corresponding spherical wave expands from m’ to the surface of a 
sphere, whose radius m’o’ is to mo, as n’k to nk. Hence it follows that all 
spherical waves, dependent upon the same incident plane wave, and 
proceeding from the different points between m and k, are tangent to one 
‘and the same plane, ko’o, parallel to which the refracted wave is propagated. 
‘As the lengths, nk and mo, are as the velocities of transmission of light 
waves in the two media, their ratio will be a constant one, or 
nk 
Pe Ts the symbol assumed for this ratio. Now we have nk = mk sin. nmk, 
and mo=mksin.mko; therefore, by eliminating mk, — substituting 
. _ nk . 
the symbol m for the ratio Fai reducing, we _ shall have 
eat =m, or sin.nmk = msin.mko. By erecting a perpendicular to 
sin. mko . 5 
mk at m, it will be readily seen that the angle nmk = the angle of incidence 
of the ray, and mko = the angle of refraction. 
A remarkable phenomenon first discovered by Grrmaxop1 of Bologna, is the 
diffraction of light, that deviation or deflection experienced by rays of light 
in passing by the edges of opake bodies. Thus, allow a solar ray to enter a 
dark room through a small aperture, and into the axis of the ray introduce 
a very thin plate of metal, with a very minute hole bored through it. If the 
light passing through this hole be received on a white screen, instead of 
a simple white spot there will be perceived one surrounded by several rings. 
If a fine slit be made in the metal plate, instead of the circular aperture, 
streaks parallel to the slit will be observed, which are alternately light and 
dark. Experiments on this subject are best conducted by examining the 
phenomena through a closely approximated telescope. 
Fig. 93, pl. 21, represents the appearance presented by looking through a 
narrow slit at a point or line of light, homogeneous light, as that produced 
by interposing a piece of red glass, being employed. In the middle is seen 
a very bright stripe, and on each side others of sensibly diminishing bright- 
ness, separated by dark intervals. These are called by Fravunnorer 
spectra of the first order. A parallelogram-shaped aperture presents the 
appearance seen in fig. 94; a circular one that in fig. 95. A few brief 
indications are all that we can here present of the explanation of these 
phenomena afforded by the undulatory theory of light. If the light from a 
distant point fall perpendicularly upon the plane of the screen AB, in which 
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