HISTORY OF PHYSICAL ASTRONOMY.

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admitting the sun's light through a very small aperture into a dark room,and then receiving the cone of light thus formed upon a metal disk of some-what larger dimensions than the circular section of the cone. Upon thenviewing the metal disk from behind, either with the naked eye or with asmall telescope, he perceived around it a bright margin of light, exactlyresembling that which was visible in the experiment of La Hire *.

The explanation of the luminous ring proposed by Delisle has beenfavourably received by several enquirers in recent times, but the attemptsto obtain an experimental illustration of it have not been attended withuniform success. In some instances the sun was artificially eclipsed byplacing an opaque disk at the focus of the telescope; but no appearanceresembling the luminous ring was discernible around the edge of thedisk. In 1840 Prof. Powell communicated to the Astronomical So-ciety, a paper containing an account of a series of experiments executedby him, with the view of elucidating this interesting subject f. He as-cribes the failure of some of the previous experiments of the same natureto the circumstance that the origin of light which was situate in the focusof the telescope was made to coincide with the opaque disk eclipsing it,whereas, in all cases of diffraction it is an indispensable condition thatthe intercepting body should be at some distance from the origin of light.Before proceeding to describe his own attempts to produce the pheno-menon of the luminous ring, he remarks that in all those experimentswhich have for their object the exhibition of the ordinary diffractive fringes,it is essentially necessary, in the first place, that the origin of light be apoint, or as nearly as possible so ; and, secondly, that the area of thediverging rays extend beyond the edge of the opaque diffracting body J.Having admitted the sun’s light through a series of apertures varyingfrom jL to j of an inch in diameter, he found that the fringes ceased tobe distinctly visible when the diameter exceeded J of an inch. Moreover,he remarked, that in all such experiments the fringes could only be seendirectly, as forming an optical image in the air, which was magnified byan eye-lens.

A phenomenon of a different kind was, however, perceptible, whichcontinued to be discerned even when the conditions of the experimentwere esentially varied. When a circular disk was employed to interceptthe light, it was seen with the naked eye, or in a telescope, at a distance,bordered by a luminous ring, even in those cases wherein the diameter ofthe aperture exceeded \ of an inch, and whether the area of the rays fellwithout or within the opaque disk. It was found also, that although thering was distinctly visible with the telescope when in focus for the opaquedisk, it could not be seen with the eye-lens. Upon this ground Prof.Powell concludes that the phenomenon of the ring is not an opticalimage like that formed by the diffractive fringes, and, therefore, that itcannot be ascribed to the principle of diffraction, at least when consideredaccording to the usual acceptation of the term.

Prof. Powell found that the ring ceased to be discernible whenthe area of the rays fell short of 4 of the diameter of the diffractingdisk, and that it increased in breadth as the area approached to an equa-

* Mem. Acad, des Sciences, 1715, p. 166, et seq.

f Mem. Ast. Soc. vol. xvi., p. 301, et seq.

t It was in consequence of employing too large an aperture that Mariotte failedto verify the experiment of Grimaldi, and was hence led to doubt the reality of theprinciple of diffraction ( Traits des Couleurs, p. 33).

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