260 FRESNEL. 
only sensible in the directions which, prolonged, ter- 
minate in the luminous points, although in each of its 
successive positions the different parts of the primitive 
wave are in fact themselves the centres of disturbance, 
whence emanate new waves in all directions ; but these 
oblique or secondary waves interfere with each other, 
and destroy each other entirely. There remain then 
only the normal waves ; and thus the rectilinear propa- 
gation of light finds an explanation in the system of 
vibrations. 
When the original wave is not entire, when it is broken 
or intercepted by the presence of an opaque body, the 
result of the interferences (which in this case play an 
important part) is not so simple to explain: the rays 
which go off obliquely from all parts of the wave not in- 
tercepted, do not necessarily destroy each other. In one 
part they conspire with the normal ray, and produce a 
brilliant light; in another these same rays destroy each 
other, and all light disappears. From the point where a 
ray is broken, its propagation is effected thenceforward 
according to special laws ; the light which falls upon a 
screen is no longer uniform: it necessarily is composed 
of alternate stripes of brightness and darkness regularly 
placed. If the opaque intercepting body is not very 
large, the oblique waves which cross each other within 
verging rays cannot be seen. This apparently paradoxical effect has . 
been supposed by some not sufficiently explained on M. Babinet’s 
principle. The reader will find some observations on the subject, and 
its applications in the author’s two papers in the Memoirs of the Royal 
Astronomical Society, vol. xvi., on Luminous Rings round Shadows, 
and in vol. xviii., on Irradiation. Some further remarks also will be 
found in his paper on Lord Brougham’s Experiments, Phil. Mag. July, 
1852.— Translator. 
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