13S Royal Astronomical Society. 



as a phsenomenon of diffraction. Several instances are then given 

 of luminous rings surrounding terrestrial objects seen in shadow, as 

 observed by different philosophers. 



Experimental imitations of the luminous ring, by artificially eclip- 

 sing the sun, were long ago made by De I'lsle and La Hiri {Mem. 

 Acad. Paris, 1715, p. 166), and such an intimation is also spoken of 

 by Mr. Baily (Ast. Soc. Notices, vol. v. p. 212). 



Among the experiments described by Newton in the third book 

 of his Optics, one, which is emphatically mentioned as unfinished, 

 seems to bear closely on the present subject. The sun's light being 

 admitted through a hole one quarter of an inch in diameter, and 

 partly intercepted by a knife-edge fixed along one side of a hole 

 three quarters of an inch square in a screen, and faUing on a paper 

 beyond, Newton saw " two streams of faint licjht shoot out both 

 ways from the beam of light into the shadow like the tails of comets." 

 Again, placing his eye to receive the light, he saw " a line of light 

 upon the edge (of the knife) all along it.". ..." It was contiguous to 

 the edge and nan-ower than the innermost fringe," and " between the 

 edge and the first fringe." Newton thus clearly distinguishes be- 

 tween this peculiar phsenomenon and the ordinary effects of diffrac- 

 tion. 



The author then proceeds to detail his experiments made for the 

 purpose of producing the luminous ring, remarking that the essential 

 conditions in all diffraction experiments are, that the origin of the 

 light be as nearly a point as may be, and that the area of the rays 

 diverging from it extend beyond the edge of the opake diffracting 

 body. 



He finds that with apertures up to a quarter of an inch or more, 

 and whether the area of the rays reach beyond the edge of the 

 opake body, or lie even considerably within it, a small circular disc, 

 seen at a distance, either with or without a telescope, is edged by a 

 bright luminous ring, which cannot be seen with the eye-lens, and is 

 therefore not an optical image like the diffraction fringes, but is seen 

 in the telescope distinctly when in focus for the opake disc. With 

 a straight edge the same phosnomenon is observed a.^; a line of light 

 running along it. If the origin be a lens of short focus the ring is 

 seen as before, but less perfectly ; while the diffraction fringes are 

 seen perfectly with the eye-lens. The origin being a hole, the ring 

 is visible under different variations of the area of the rays, but ceases 

 to be visible when the area is less than about a quarter of the disc. 

 Changes in the distance of the eye from the disc produce very little, 

 if any, change in the breadth of the ring. 



The ring was examined by a telescope with cross-wires in its 

 focus. The intersection of the wires was fixed on the edge of the 

 disc before the liglit was admitted, and on its admission the ring 

 extended sensibly beyond ; on its being then fixed on the edge of 

 the ring, the edge of the disc fell within it on shutting out the light. 

 The experiment was repeated in various ways to determine the in- 

 fluence of reflexion from the edge, &c. 



When the area is a hole, the disc being -within the area of the 



