Researches in Circular Solar Spectra. Ill 



central disk now became pearly white, set off prettily by its com- 

 panion black ring and a number of pale lavender, rose colour, and 

 then brilliant outer circles of bright green with intervals of orange 

 red, and more outwardly, 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 fragment of cover, • 003 inch thick to the eighth, and deli- 

 cately focussed 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 difierent planes of vision, or dif- 

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

 magnificent rings (including the black rings and interspaces) dis- 

 played at one time in the same field of view. Derived directly from 

 the sun, with the brilliance belonging to total internal reflexion, 

 this rich assemblage of gorgeous rings, rivalling each other in pris- 

 matic 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 difii-action. 



Doubtless, however, these appearances surpassed in intensity and 

 brilliance those he described. (PI. XXXIII., Figs. 2, 3.) 

 Careful measurements were made. 

 The diameter of the central disk, xt^st' 

 Breadth of its black ring. 



50000- 



My surprise was further increased by observing that, by length- 

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

 the ruled lines of a micrometric eye-piece (200 to inch) to coin- 

 cide exactly with the inner edge of each black ring ; so that the 

 breadth of each complete ring was exactly the same as that of the 

 central disk, viz. nriaa of an inch. (PL XXXIIL, Fig. 3.) 



Slight changes in the colours of the rings were caused by the 

 use of the Eoss collar corrections. 



In these researches a very near approximation to achromatism 

 was signified by the whiteness of the central disk ; the blackness of 

 the fine rings contrasting finely with the intervening rings which 

 were then of a lavender grey, or very pale and yet brilliant 

 lavender. 



Destruction of spherical aberration appeared imminent, when the 

 rings, still coloured, were tolerably symmetrical on difierent sides 

 of the finest focus with contrasting colours of the residuary spec- 

 trum. Mechanical errors were displayed by irregularity and com- 

 plexity of form. 



