430 



Dr. Royston-Pigott on 



[June 19, 



In order to determine the cause of this and to insure one axis in the 

 solar disk, I substituted for the miniature-giving objective a convexo- 

 plane lens of half an inch focus ; and as this gave too large a disk, the 

 lens of the heliostat was reduced from three inches to an inch and a half 

 focal length. 



[Unless the solar disk is reduced, the splendid phenomena of the rings 

 cannot be properly developed. Their number and colours change with 

 the slightest change of the plane of focal vision ; and a very fine and 

 delicate focal adjustment-screw, as well as great firmness in the apparatus, 

 are essential to a successful display of the rings in all their wonderful 

 beauty and complexity.] 



The disk formed by the simple lens was now scrutinized with the Powell 

 and Lealand celebrated immersion " eighth." Deeper eyepieces and a 

 lengthened tube were employed to subdue the intolerable brilliance of 

 the coloured rings ; they now exactly filled the whole field of view. At 

 first, used dry (improperly), this objective displayed a crimson solar disk 

 edged with an intensely black ring encircled with a much broader bright 

 ring, resembling the planet Saturn viewed perpendicularly to the plane of 

 his ring. (PI. II. figs. 1-3.) 



Deepening the focus with exceeding lightness of touch, the central 

 disk now became pearly white, set off prettily by its companion black 

 ring and a number of pale lavender, pale rose-colour, and then brilliant 

 outer circles of bright green, with intervals of orange-red, and, more out- 

 wardly, circles of red merging into ill-defined black. 



But as the glass was constructed for vision through a film of water 

 upon a thin glass " cover," I now attached (by moisture) a small frag- 

 ment of cover, 0*003 inch thick, to the eighth, and delicately focused 

 down upon the solar spectrum. 



The solar disk then appeared single, circular, and bounded by a clear 

 sharp black edge almost perfectly circular. 



Upon examining the axis in different planes of vision or different 

 sections of the solar pencil, I counted no less than forty-eight magnifi- 

 cent rings (including the black rings and interspaces) displayed at one 

 time in the same field of view. Derived directly from the sun, with the 

 brilliance belonging to total internal reflection, this rich assemblage of 

 gorgeous rings, rivalling each other in prismatic splendour, set off by 

 the sharp contrasts of jet-black well-defined borders, and shaded with the 

 most delicate tints melting into one another with an exquisite softness, 

 reminded me of the eloquent and glowing language of the late Sir John 

 Herschel when describing the phenomena of diffraction. Doubtless, 

 however, these appearances surpassed in intensity and brilliance those he 

 described. (PI. II. figs. 2 & 3.) Careful measurements were made. 

 The diameter of the central disk y^-J-g-g-; breadth of its black ring 

 50 ioo* 



My surprise was further increased by observing that, by lengthening 

 the tube to increase power, I was enabled to cause each of the ruled 



