1873.] 



Circular Solar Spectra. 



431 



lines of a micrometric eyepiece (200 to an inch) to coincide 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.y^-ly-g- £ an 

 inch. (PL II. fig. 3.) 



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

 Ross 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 different sides of the finest 

 focus with contrasting colours of the residuary spectrum. Mechanical 

 errors were displayed by irregularity and complexity of form. 



I shall now venture to give some particulars of the circular solar 

 spectrum thus formed (as described) by a convexo-plane lens in a beam of 

 sunshine examined by a high-quality immersion-objective armed with a 

 small piece of broken cover attached by the cohesion of water. 



Description of the Rings of the Circular Spectrum of a 



Convexo-plane Lens. 

 Coloured rings. Intervals. 



Solar disk 



. White. 



Primary ring . . . 



. Jet-black. 



Eing IT 



. Pale lavender. 



Secondary ring 



. Black. 



„ IV. ... 



. Lavender. 



Third ring .... 



55 



„ VI. ... 



• 55 



Fourth „ .... 



55 



„ VIII... . 



55 ' 



Fifth „ 



55 



„ x 



. Pale rose. 



Sixth ,, 



. Dark red. 



„ XII. ... 



. Bright green. 



Seventh,, 



55 



„ XIV.... 



55 



Eighth „ 



55 



„ XVI. ... 



55 



Ninth „ 





„ XVIII. . 



55 



Tenth „ 



. Black. 



„ XX. ... 



. Dark orange. 



Eleventh ring . . 



5~5 



„ XXII. . 



. Deep ,, 



Twelfth „ . . . 



55 



„ XXIV. . 



55 55 







Each of the rings, including the companion black or dark rings, 

 appeared exactly of the same breadth, viz. 61 millionths of an inch, or 

 nearly double the length of the wave of the extreme red ray ; whilst 

 the breadth of the primary black ring was nearly that of the wave-length 

 for the line F in Fraunhofer's spectrum, viz. 0*0004606 millimetre, or, 

 since the French metre is in English inches 



39-37078984, 

 it corresponds to 52256 waves to the English inch. 



