44 Transactions of the Society. 



flowing out from it with some modification into light rings having 

 the order of their colours inverted or the red inwards and blue 

 outwards, and each change to white at the centre producing an 

 additional prismatic ring. Fraunhofer has accurately described 

 the phenomena of the former series and Sir J. Herschell of both, 

 my memoranda on them being thus as I found anticipated. The 

 appearances of the object wlien correctly in focus are, however, 

 those only that belong to the present purpose. On measuring 

 these I found, as Fraunhofer had discovered before me, that "the 

 diameters of the rings are inversely as those of the apertures form- 

 ing them " ; a law from which it appeared to me that the prob- 

 ability at least of that of distinctness above given is fairly 

 deducible, for it seems reasonable to infer that any two points so 

 near each other that the centre of the one's image shall coincide 

 with the other's first dark ring will be very near the place at 

 which they will appear as one spot. 



There is a difference between Fraunhofer's measurements of 

 the rings and mine, which I should hesitate to state if mine had 

 not been gone over repeatedly. He gives as the semidiameter of 

 the red circle edging the central white spot (and therefore smaller 



than the first dark ring) , calling 7 the breadth of the 



7 . 

 aperture in fractions of a Paris inch, which is equivalent to 



, a being my expression for the diameter in English inches, 



and he makes the space between the first and second red ring 



0000214 / 0000228\ , , ^ . , ^ .1 • ^ • .v. 



and between the second and third rmj? the 



7 \ a J 



same. The objects measured by me were the reflexions of the 

 sun from globules of mercury seen in the achromatic Microscope, 

 the globules being of various sizes but all so small that the sun's 

 image might be considered as a point but for the effect of diffrac- 

 tion. They were first viewed with a double object-glass of 1\ in. 

 focus, and afterwards other achromatic lenses were added in front 

 of it, which brought the focal length to ^q in. without producing 

 any change whatever in the size of the rings, the apertures behind 

 the object-glass, i.e. the diameter of the inflected pencil, continuing 

 the same. Their dimensions were taken with a very accurate 

 micrometer, and also by tracing them on paper with a camera- 

 lucida. The mean of the observations with the micrometer 

 was : 



From the centre to the middle of the first dark rins^ 



0000245. 

 a 



