MICROSCOPICAL IMAOERY. 107 



discs. A one-sixteenth water immersion of Powell and Lealand, 

 displayed two overlapping spurious discs, each forming its own in- 

 dependent diffraction-systems above the best focus and evanish- 

 ing below it with a confused bright halo. 



It seems extraordinary that a simple piano convex lens, form- 

 ing the miniature of the sun, emitting a very minute beam, should 

 be capable of testing severely the qualities of observing objectives. 



A very excellent water immersion, one-tenth, was armed with a 

 small piece of thin glass attached by water to the front lens, and 

 focussed upon the spectrum. Deepening the focus with exceeding 

 lightness of touch, the central disc became snowy or pearly white, 

 set off prettily by its companion black ring and a number of pale 

 lavender, pale rose colour, and then brillant outer circles of bright 

 green with intervals of orange-red, and more outwardly still circles 

 of red merging into ill-deftned black. By this diffracting solar 

 ]:)encil, the precision of the construction of this fine glass was thus 

 revealed by the use of a simple convex lens of crown glass of half- 

 an-inch focus. '^ 



As the axis of this single lens was necessarily unique, the phe- 

 nomena, especially the sharpness and intensity of the jet black 

 ring, could only be so superbly exhibited by very fine glasses. 

 Inferior glasses blurred them, and utterly marred the rich and 

 delicate beauty of the colours. Contrasting a variety of English 

 and foreign glasses, many appearances arose, which indicated grave 

 errors of construction, too numerous for insertion here. The 

 secret for accomplishing these most extraordinary glories, is once 

 for all the employment of a very minute beam of solar light, 

 emanating from a small orifice as aforesaid, a few yards distant. In 

 the focus of a powerful microscope,t brilliant coloured rings were 

 developed by a half-inch convex plane lens. Upon examining the 

 axis in different focal planes of vision {i.e., different sections of the 

 conical solar pencil), I counted no less than 48 magnificent rings, 

 including black rings and interspaces, at one time in the same 

 field of view (Fig. 3). Derived directly from the sun, with the 

 brilliance belonging to total internal reflection, this rich assemblage 



* The eye-lens of a Huyghenian eye-piece. 



t Sir (i. Airy informed me he had seen a few of these with a |)ocke( lens. 



