[ 106 ] 



fIDicroecopical 3maocrv\ 



By Dr. Rovstox-Pigott, M.A., F.R.S. 



Solar Splendours. 

 Plate X. 



TO obtain heliostatic diffractions of the highest order, an ex- 

 ceedingly small beam of light must be attained from a 

 distant luminous source. (See " Circular Solar Spectra, Pro- 

 ceedings of Royal Society.") 



In some cases, eye pieces were placed to catch the solar beam, 

 also objectives. Plane mirrors, silvered at the back, utterly failed. 

 I was therefore driven to dei)end upon the truth of the surface of 

 the prism, acting by internal reflection. To show the exceeding 

 delicacy of the experiments : If two piano convex lenses are placed 

 with coincident axes on the stage, a good many coloured rings can 

 be seen, but no black ones. So soon as their axes become in the 

 least degree oblique, the solar spectrum takes an intricate form, 

 whilst the centre shows a brilliant black Maltese cross. Very 

 worthy of the highest photographic art, are all the rich forms 

 displayed. 



Supposing only one piano convex lens is used, the most striking 

 appearance, amid such splendid effulgence, is a very thin^ intensely 

 jet black ring enveloping the small brilliant central disc. The rings 

 change their appearance every instant with the most minute altera- 

 tion of the focal planes. 



On first viewing this amazing glory, my attention was arrested 

 by the shape of the primary black ring being squared off. Upon 

 severe examination, I found it was composed of several excentric 

 rings, which no change of collar corrections or length of body at 

 all ameliorated. Another fact rather surprised me : " collar adjust- 

 ment " entirely, in some glasses, decentred the solar disc, so that 

 two discs appeared occasionally instead of one — the sign of inferior 

 workmanship. (See Plate X., Fig. 4.) 



An Andrew Ross "quarter" (1851), displayed several irregu- 

 larly placed central discs, which formed so many different centres 

 of diffraction rings. A Berlin glass (Gundlach) showed a much 

 finer ]:)rimary black ring ; but in a deeper focus I saw four spurious 



