^ Minute Organic F articles. 17 



particles of brilliant reflecting and refracting properties are huddled 

 together ; either under known or unknown arrangements in different 

 strata. 



The fourth, the nature of mixed shadows. 



Fifth, the nature of perfect definition. 



As the present paper is intended to be rather suggestive than 

 demonstrative, I propose to treat very briefly these several points : 

 not for the consideration of those whose optical education- is complete, 

 and their views unalterably matured, but for that more numerous 

 class who have not the leisure to make these subjects a special study. 



I. Circle of least confusion. 



(1.) No lens, or combination of lenses yet made, can form a 

 perfectly correct image of a brilliant point. If you place a small 

 plano-convex lens on the stage and view the image of a miniature 

 sun formed by it, the first glance will convince you such a thing is 

 impossible. 



Examine. — I formed the image of a miniature sun, by optical 

 means ^^ ,yth of an inch in diameter, upon a distant table. Again 

 by means of a J- inch plano-convex lens, another image was obtained 

 on the stage, and viewed with a very excellent objective. I then 

 measured the solar disk, and found it enormously larger than it 

 ought to be, even at the best focus. On diminishing the primary 

 image almost to a point, the solar disk still retained nearly the same 



similar to Fig. 1, viz. elongations with jet-black shaded edges, without heads and 

 tails, which are spurious. They become more and more continuous. In the actual 

 photograplis of the beaded forms, Fig. 2 and Fig. 4, it is almost impossible to decide, 

 except by the designation written upon them, which is the Degeeria and which the 

 Curvicollis. Fig. 3 ought, therefore, to more nearly aiJi^roximate to Fig. 1 in the 

 manner just described. 



I have ventured to insert some faint white beading within the circle drawn at 

 Fig. 3, as seen with the one-fiftieth immersion, as described at page 25 ; but in 

 this case the " tadpoles " had vanished heads and tails, and were transformed into 

 apparently continuous ribs. But it did not seem advisable to alter further Mr. 

 Hollick's very exact and faithful representation of these splendid sun-pictures 

 from monochromatic light. Figs. 1 and 2 are 2300 diameters. Figs. 3 and 4 

 3200 diameters. All of them were photographed with Powell and Lealand's 

 immersion sixteenth of their new construction of the winter of 18G9, and sent to 

 America in December of that year. 



The tadpoles are merely the result of the crossing obliquely of the two sets of 

 bearded rouleaus which, intersecting at somewhat irregular angles, cause them to 

 occupy an irregular natant position in the picture. 



I have obtained some beautiful results witli my Aplanatic Searcher and a 

 Gundlach " one-sixteenth " of excellent quality, which would have been finer still 

 had the lenses of their objective been more perfectly centred. 



This glass is deeper than Powell and Lealand's one-eighth in the proportion of 

 800 to 1800. It may therefore be called a thirteenth, magnifying 1300 diameters 

 with the C eye-piece and the front lenses screwed home. 



iVofe.— Since writing this notice a month ago, I have (June 12, 1873) for- 

 tunately succeeded in forming a new correction, which has surpassed all former 

 excellence of High-power definition as observed by me. 



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