176 Besearclies in Circular Solar Spectra. 



The frequent appearance of several disks at once in the field of 

 view caused me to suspect that the axes of the component lenses 

 of the objectives were not always coincident. 



An Andrew Koss "quarter" marked 1851, though of good qua- 

 lity, displayed several irregularly-placed central disks which formed 

 so many different centres of diffraction-rings. 



A Berhn glass of good quahty showed a much finer primary 

 black ring, and a splendid display of several coloured rings edged 

 with black ; in a deeper focus four false centres appeared. 



It will be not out of place here to detail a few experiments con- 

 ducted with the object of verifying the cause of irregularity in the 

 primary black, and the particular signs of chromatic and spherical 

 residuary aberration. 



One of the finest " immersion " one-eighths of Powell and Lea- 

 land, made expressly for me in 1871, the aberrations of which were 

 small compared with those of glasses of their old construction, was 

 now used to form the miniature solar disk on the stage, as derived 

 from the distant prism-heliostat. 



I then examined the solar disk with a Powell and Lealand one- 

 sixteenth immersion objective (1862 make) adapted to water in 

 1870, a water-film being introduced between the glasses whose 

 axes had been carefully adjusted to coincidence. 



Tivo overlapinng disks were now seen. Each formed its own 

 independent diffraction systems above the best focus, and evanished 

 below it with a confused bright halo. 



In order to determine the cause of this and to ensure one axis 

 in the solar disk, I substituted for the miniature-giving objective a 

 convexo-plaue lens of half an inch focus ; and as this gave too large 

 a disk, the lens of the heliostat was reduced from three to an inch 

 and a half focal length. 



[Unless the solar disk is reduced, the splendid phenomena of 

 the rings cannot be properly developed. Their number and colours 

 change with the slightest change of the plane of focal vision, and a 

 very fine and delicate focal adjustment screw, as well as great firm- 

 ness in the apparatus, are essential to a successful display of the 

 rings in all their wonderful beauty and complexity.] 



The disk formed by the simj^le lens was now scrutinized with 

 the Powell and Lealand celebrated immersion " eighth." Deeper 

 eye-pieces and a lengthened tulDC were employed to subdue the in- 

 tolerable brilliance of the coloured rings. They now exactly filled 

 the whole field of view. At first, used dry (improperly), this objec- 

 tive displayed a crimson solar disk edged with an intensely black 

 ring encircled with a much broader bright ring, resembling the 

 planet Saturn viewed perpendicularly to the plane of his ring. 

 (PI. XXXIII., Figs. 1-3.) 



Deepening the focus with exceeding lightness of touch, the 



