260 FRESNEL. 



only sensible in the directions which, prolonged, ter- 

 minate in the luminous points, although in eacli 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 eftected 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. 



