110 MICROSCOIMCAI. I.MA(;E1!V. 



spectra, I found silvered mirrors useless, but the internal reflections 

 of a distant prism fully succeeded. The illuminated prism helio- 

 stat, is roughly delineated in the engraving, and when seen, has 

 been mistaken for the electric light. 



The aerial solar image, loo inches distant, formed by the 

 prism armed with a 3-inch lens, was i-4oth of an inch in diameter. 

 A small piano convex lens placed truly on the " stage,'' now further 

 reduces the image and dims the effulgence. On focussing with 

 1000 diameters upon the solar miniature, cruel revelations are 

 instantly made. Imperfect curvatures and centering of the 

 observing objectives are at once made manifest. These errors are 

 egregiously displayed. The central rings are irregular in outline, 

 confused, and e.xcentric. (Figs. 5, 6, and 7.) 



(To be continued.) 



EXPLANATION OF PLATE X. 



Fig. L — Primary rings tiuely detiued at the first visible focal plane. 



,, 2. — Secondary rings at a deeper focus. The central discs should have 

 been s«iuared off. 



,, -3. " Exhibition of the greatest display of rings, each annul us having its own 

 V)readth e(|ual to that of the central disc, which is brilliant white ; 

 the succeeding, lavender, rose colour and red rings, separated by 

 dark rings, the first few of which are jet black. 



.. 4, ."», tj, 7. — Development of two discs instead of one ; also of four irregular 

 discs showing the existence of displaced centres ai.d irregular 

 diffractions. 



