1873.] 



Circular Solar Spectra. 



429 



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 oppo- 

 site 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 primary disks, each with 

 its own system of rings, struggled for the mastery ; and on changing the 

 focus a chromatrope effect was produced by the expanding rings and 

 their excentric intersections presenting an extraordinary loveliness of 

 colours. 



Another result somewhat startled me. In some of the best glasses 

 the movement of the Ross 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 setting, are preferable, 

 especially as a new compensation can be effected, as described p. 435*. 



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 Eoss " quarter," marked 1851, though of good quality, 

 displayed several irregularly placed central disks, which formed so many 

 different centres of diffraction-rings. 



A Berlin glass of good quality 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 not be out of place here to detail a few experiments conducted 

 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 Lealand, 

 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 overlapping disks were now seen : each formed its own inde- 

 pendent diffraction systems above the best focus, and evanished below it 

 with a confused bright halo. 



* By means of the compensating eyepiece. 



