1873.] 



Circular Solar Spectra. 



427 



generally with spherical aberration, require also particular attention. I 

 have repeatedly observed a fine state of definition completely blurred 

 merely by a change of eyepiece of the same power, which no mere 

 focusing ameliorated, and which could only be corrected by a change 

 in the convergent pencil passing through the objective intrinsically 

 affecting its aberration. 



Another branch of such an inquiry would be the nature of the definition 

 of an organic particle under high poivers, as every such research (such as 

 the detection of the characteristics of diseased and healthy cells) may be 

 resolved into the power of the microscope to define a single organic 

 particle. Such particles are generally brilliant and refracting, and the 

 errors of observation are unfortunately at present of a numerous kind, 

 (Appendix A.) 



On the Circular Solar Spectrum. 



If a lens* be placed with its axis coincident with that of the micro- 

 scope, and if its principal focus formed by the solar rays be examined by 

 an instrument of the highest quality, we shall find that minute slices, as 

 it were, of the solar cone present phenomena of rare beauty and order, 

 dependent upon the quality of the examining instruments. 



If two plano-convex lenses are placed with axes coincident, a good 

 many coloured rings may be counted, but no black ones ; so soon as their 

 axes become oblique, the solar spectrum takes an intricate form, whilst 

 the centre shows a brilliant cross ( % ), very difficult to describe or repre- 

 sent by portraiture, but worthy of the highest photographic art of all 

 the forms described. 



This spectrum I venture to name the circular solar spectrum. 



In my former research I had observed a flame-disk in a darkened room. 

 This disk presented two or three diffraction-rings, similar to those of 

 telescopic stars, but much broader. 



The same method was attempted with the sun. Various objectives 

 were used to obtain a solar miniature of the sun's disk. 



Plain mirrors of glass silvered at the back entirely failed. 



In order to form a pure and brilliant solar spectrum under the micro- 

 scope, it occurred to me to take advantage of the principle of total internal 

 reflection from a prism. I then constructed aprism-heliostat, which, acting in 

 sunshine, presented an aerial miniature of the sun of great splendour (almost 

 as dazzling as the sun itself) for half an hour, without further adjustment. 



The prism-heliostat was furnished with a crown-glass double-convex 

 lens, itself being of flint ; and other lenses of less focal length could be 

 attached to diminish or increase the diameter of the primary image of the 

 sun, which, as the focal length generally used was 3 inches, gave an 

 image 3 sin 30' in diameter, or one fortieth of an inch nearly. In some 

 cases object-glasses and eyepieces were placed in the solar rays emanat- 

 ing from the prism. 



* This term of course includes every form. Of concave lenses, however, only vei*y 

 small ones can be conveniently examined. 



