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 and continuing its motion, it meets the ray with which it before interfered, its relations with this as to intervals will have been altered; and if the intervals are ever found to be the same, it must be when the ray is so refracted by the glass that the diminution of its velocity be compensated by shortening its path; in this case the fringes will be formed in different places, and their displacement may be calculated from the thickness of the glass and its refracting power; now this is confirmed by experiment with incredible exactness, as M. Arago observes, to whom we are indebted for this ingenious experiment.

By the same rule, if the displacement of the fringes thus produced by a given plate be observed, which may be done with extreme precision, we may evidently find the refracting power of that plate; we may also compare the refractions of various substances by interposing plates of them successively on the directions of the interfering rays. Messrs. Arago and Fresnel tried this method, and found it so exact that they were able to use it to measure differences of refraction that no other method would have given.

When a beam of light is introduced into a dark room, if you place on its direction the edge of some opaque body, and afterwards receive on a white surface placed at a certain distance that portion of the light which is not intercepted, the border of the shadow will be observed to be edged with a bright line; and on increasing the distance, several alternations of coloured fringes are thus seen to be formed. This phænomenon constitutes what is called the Diffraction of light.

To give it all the exactness of which it is capable, it is advisable to use the same disposition as in the experiment with the two mirrors, that is, to take a sunbeam directed by a heliostat and concentrated by a lens almost into a geometrical point: an opaque body is then to be placed in the cone of rays diverging from that point. To fix our ideas, suppose we use an opaque lamina with straight edges, and about a tenth of an inch broad; if then the rays be received on a piece of ground glass placed at a certain distance, and the eye be placed beyond this glass, there will be observed on each side of the shadow of the lamina a numerous