Eesearehes in Circular Solar Spectra. 173 



the effects of the interference of complex cones of light of large 

 angular aperture, at least twenty times larger than those observable 

 by the telescope. 



The subject of the optical contacts of Venus at the coming 

 transit confers peculiar interest on the nature of accurate definition 

 of the final image presented to the eye-glass, especially as the new 

 parallax will be entirely dependent on the keen definition of the four 

 contacts. 



The discovery by the writer of an unsuspected residuary aberra- 

 tion in the best microscopes, described in the ' Philosophical Transac- 

 tions ' for 1870, renders it probable that some such a residuum stiU 

 remains in telescopes ; and this might impair the accuracy of such 

 delicate observations as the apparent contacts between Venus and 

 the solar limb. Eye-pieces, abounding generally with spherical 

 aberration, require also particular attention. I have repeatedly 

 observed a fine state of definition completely blurred merely by a 

 change of eye-piece of the same power which no mere focussing 

 ameliorated, and which could only be corrected by a change in the 

 convergent pencil passing through the objective intrinsically affect- 

 ing its aberration. 



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

 definition of an organic lyarticle under high ^mwers, 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.* 



On the Circular Solar Sjjectrum. 



If a lens t be placed within its axis coincident with that of the 

 microscope ; and if its principal focus, formed by the solar rays, 

 be examined by an instrument of the highest quahty, we shall find 

 that minute slices, as it were, of the solar cone present phenomena 

 of rare beauty and order dependent upon the quahty of the examin- 

 ing 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 (fi^) very 

 difiicult to describe or represent by portraiture, but worthy of the 

 highest photographic art are all the forms described. 



This spectrum I venture to name the circidar solar spectrum. 



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



* Appendix A. 



t This term of course includes every form. Of concave lenses, however, only 

 verv small ones can be conveniently examined. 



p 2 



