178 



Besearches in Circular Solar Spectra. 



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

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

 beam of sunshine examined by a high-quahty 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 



CONTEXO-PLANE LeNS. 



Coloured Rings. 



Intervals. 



Solar disk 

 Eing II. 

 „ IV. 



White 



Pale lavender 



Lavender 



VI. 



VIII. 



X. 



XII. 



XIV. 



XVI. 



XVIII. 



XX. 



XXII. 



XXIV. 



Pale rose 

 Bright green 



Dark orange 

 Deep „ 



Primary ring 

 Secondary rinj 

 Thii-d 



Fourth „ 



Fifth „ 



Sixth „ 



Seventh ,, 

 Eighth 

 Ninth „ 



Tenth „ 



Eleventh ,, 

 Twelfth 



Jet-black. 

 Black. 



Dark red. 



Black. 



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

 appeared exactly of the same breadth, viz. 61-miUionths 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 hne F in Fraunhofer's spectrum, -yiz. 0" 0004606 

 millimetres, or, since the French metre is in EngHsh inches, 



39-37078984, 



it corresponds to 52,256 waves to the Enghsh inch. 



The delicate measurement of the primary black ring and disk 

 was verified by a recording eye-piece micrometer. With this, and 

 the objective used, it was found that one-thousandth of an inch on 

 the stage measured 1138 divisions, i. e. eleven turns of the divided 

 head and 38-lOOths of a turn. One division therefore represented 



0-001 



1138 1,138,000 



nine ten-millionths nearly. 



On estimating the breadth of the primary jet-black ring, and 

 using a ruled glass micrometer, as I could detect no difference in 

 the breadths of each ring, I felt justified in dividing the total 

 diameter by the number of rings in order to obtain the breadth of 

 one which gave T^^iyg- A much deeper point in the axis showed a 

 very deep blue central haze, paling outwardly, and then melting 

 into a final red fringe. 



