Researches in Circular Solar Spectra. 179 



The precision of the mechanical construction of this fine ob- 

 jective "was thus revealed by the use of a simple convex lens of 

 crown glass. Any deviation from accuracy was at once detected 

 by the converging pencil of the plano-convex lens, consisting of 

 shells of rays of various refrangibilities, having their several foci 

 arranged along the axis. As this axis was necessarily single and 

 unique, the interference phenomena, especially the sharpness and 

 intensity of the jet-black rings, could only be so superbly exhibited 

 by the best glasses. Inferior glasses bluiTed them and dulled the 

 rich beauty of the colours. 



Contrasting with this the performance of a variety of glasses, 

 both English and foreign, very peculiar appearances arose which 

 doubtless indicated grave errors of construction. I will venture 

 briefly to mention some of these : — 



(I.) A variety of spurious disks oddly arranged were displayed. 



(II.) The beauty of the rings was entirely marred ; and 



(III.) Very few rings could be developed, and sometimes no 

 black rings whatever. (PI. XXXIII., Fig. 7, and PI. XXXIV., 

 Fig. 14.) 



(IV.) Notched, grained, and spotted ; the rings were sometimes 

 irregular in shape (Fig. 12). 



(V.) A multitude of fine black eccentric rings, evidently arising 

 from different centres, were seen upon a leaden-grey field sur- 

 rounding the central disk, the confusion of the rings causing a bad 

 achromatism. 



(VI.) An "engine-turned pattern" was not unfrequent, dege- 

 nerating into a peculiar grained and mottled appearance. 



(VII.) A majority of the glasses were over-corrected spherically. 



(VIII.) Achromatism and aplanatism in our best adjustable 

 glasses were found to be altogether incongruous. 



As this result was alluded to at page 175, I proceed to relate 

 the circumstances of the observation. I constantly found in pur- 

 suing these researches that either achromatism was sacrificed to 

 aplanatism, or that the attainment of achromatism destroyed the 

 brightness and truth of aplanatism. I may relate the following 

 experience. 



Whenever in watching the hehostat the sun was clouded over, 

 the microscopic miniature-perspective of the room and distant 

 apparatus reappeared ; and after various adjustments I obtained 

 a perfect definition free from mist, and as clear and sharp as that of 

 an opera glass. The prism and lens of the heliostat then gave a 

 pretty picture of the passing clouds, as well as the small details of 

 the distant objects ; but the instant the sun began to shine, before 

 the rings dazzled the sight, every shining point appeared haloed 

 with a corona of orange and red. I now turned aside the prism : 

 then every polished point in the full sunshine exhibited the same 



