428 Dr. Royston-Pigott on [June 19, 



Keceived by an inverted object-glass of the finest quality at a distance 

 of 200 inches, the solar disk could be further miniatured to any de- 

 sirable degree , of minuteness. A theoretical diameter of sixteen mil- 

 lionths was found convenient. To moderate the overpowering brilliance 

 of such a spectrum directly viewed, dark slides of graduated neutral 

 tints were at first used, and smaller primary disks were obtained by using 

 deeper lenses at the prism. 



When this minute spectrum is viewed with a high power (800 or 1000), 

 the phenomena attending residuary errors, whether of achromatism, 

 spherical aberration, or mechanical construction, are demonstrated with 

 so keen a severity upon the handiworks of man as to throw all other 

 methods into the shade. 



Phenomena observed. 



With the flame-disk formerly used * only two or three diffraction-rings 

 could (as already remarked) be descried ; an extraordinary number of 

 richly coloured rings of dazzling brilliance was now exhibited at the in- 

 stant of bringing the solar disk into the plane of focal vision. 



The most striking feature, amid so much effulgence, was an intensely 

 black (jet-black) diffraction-ring encircling the central disk at the 

 clearest focal point. The appearance of the rings changed every instant 

 with the slightest change of focus, and their tints indicated the nature of 

 the " secondary spectrum." 



Upon closer inspection, my curiosity was excited by observing 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 examination, by change in the 

 eyepieces, length of body, and " collar corrections," that it was composed 

 of several excentric rings. The research, as one depended upon the 

 fitful gleams of a spring sunshine, though tedious, was at this point 

 enlivened with the outcome of an important fact, which will be more 

 fully noticed further on [viz. that achromatism and aplanatism, in the 

 best adjustable microscopes, at present were found to be altogether incon- 

 gruous]. 



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 sizef. 



* Phil. Trans, part ii. 1870, p. 595. 



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



Diameter of miniature at 200 inches distance, reduced by |-objec- 



tive 1600 times =0 000016 



16 



lOOOOOO* 



Observed diameter of solar disk, exclusive of its jet-black diffraction- 

 ring =0 000061 



Estimated breadth of black ring =0*00002 



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



