Researches in Cireular Solar Spectra. 175 



the shape of the primary or central black ring deviating from a 

 true circular form, somewhat squared off as though not consisting 

 of one pure black ring. I then found, upon more careful examina- 

 tion, by change in the eye-pieces, length of body and " collar 

 corrections," that it was composed of several eccentric rings. The 

 research, as one depended upon the fitful gleams of a spring 

 sunshine, though tedious, was at this pomt enlivened with the 

 out-come of an important fact which will be more fully noticed 

 farther on [viz. that achromatism and aplanatism,* in the best 

 adjustable microscopes, at present were found to be altogether 

 incongruous.] 



The same result, the enlarged disk, as described in my former 

 paper, was obtained. 



The theoretical disk (exhibited on the stage of the microscope) 

 appeared increased to nearly four times its proper size.t 



Three general features were constantly observed. The rings 

 were seen either wholly or chiefly on one side of the solar disk, 

 i. e. either within or without the focus, or nearly similar, except in 

 colour, on opposite sides of the focal point. 



If the rings were on one side a nebulous brightness occupied 

 the other, into which the solar disk suddenly resolved itself on a 

 slight change of focus. But frequently this nebulosity assumed a 

 fine-grained or " engine-turned pattern." Occasionally two pri- 

 mary disks, each with its own system of rings, struggled for the 

 mastery ; and on changing the focus, a chromatrope-effect was pro- 

 duced by the expanding rings, and their eccentric intersections, 

 presenting an extraordinary loveliness of colours. 



Another result somewhat startled me. In some of the best 

 glasses the movement of the Koss collar adjustment for the position 

 of the front lenses entirely decentred the solar disk, so that here 

 two appeared occasionally instead of one. 



This phenomenon compelled me to infer that in many cases the 

 collar adjustment may become a greater source of error than the 

 " thickness " of glass cover which it is intended to compensate ; and 

 that therefore excellent glasses, constructed with a permanent set- 

 ting, are preferable, especially as a new compensation can be effected 

 as described, p. 1824 



* Freedom from Spherical aberration. 



t Diameter of disk at prism 3 sin 30' = '026 



Diameter of miniature at 200 inches distance, reduced by| _ q.qoOOIG 



i-objective 1600 times / 



16 



1000000 ■ 



Observed diameter of solar disk, exclusive of its jet-black| ^ 0-000061 



diffraction-ring j 



Estimated breadth of black ring =0-00002 



And 0-000016 : : 0-000061 : : 4 : 1 nearly. 

 X By means of the compensating eye-piece. 



