24 On the High-Power Definition of 



the light is sent down the tube in the reverse direction, should cross 

 the axis at exactly the same precise point. That is achromatism. 



It is not generally understood that each coloured ray has its 

 own peculiar spherical aberration. 



Suppose, for instance, a blue ray from a monochromatic light 

 apparatus passes through a double convex lens, still at a focus the 

 blue marginal rays will cut the axis nearer to the lens than the 

 central blue pencils ; that is spherical aberration for a blue ray. 

 It would be much easier to construct object-glasses for use by one 

 colour — hhte, for instance; because no achromatism would be re- 

 quired ; nothing but the correction of spherical aberration of the 

 blue ray would then be requisite. 



At present, in all objectives used in compound light, however 

 fine their quality, absolute or at least almost j)erfect achromatism 

 can only be obtained at the cost of a small residuary spherical 

 aberration. Frequently, without altering the glasses intrinsically 

 (I mean without substituting other lenses of different curve, focus, 

 and index), colour can be got rid of by change of distance and 

 position. 



But as colour disappears the other residuum puts in its un- 

 welcome appearance. Nothing shows this more clearly than the 



CIKCULAR SOLAR SPECTRUM. 



Even in seeing Mr. Slack's silica cracks in his ingenious colloid 

 silica slides, these cracks, as I once formerly remarked, appear 

 often like coloured cylindrical threads; indeed the first time, he 

 laughingly decoyed me into saying they were some vegetable fibre, 

 BO like to this appeared they with the Eoss glass at the Society's 

 rooms. But these cracks can be shown black with my Searcher. 

 Nothing more particularly demonstrates the power of secondary 

 correction behind the objective than this veritable effect. 



The characteristics of S23lendid definition can be best seen in the 

 use of transparent objects of a dehcate structure suitable for a lower 

 power. When, for instance, shall we ever see the edge of the 

 P. angulatum with a high power as sharply, clearly, and briUiantly 

 displayed as the details of the fly's tongue, upon which low objec- 

 tives are usually tested ? I bought a wonderfully fine inch-and-a- 

 half from Koss, which bore the deepest eye-piece admirably. But 

 then it was carefully chosen from many. I tested it upon the inter- 

 section of two fine hairs: and the four black markings caused by the 

 interference. The same thing can be seen more beautifully with 

 the cylindrical threads of the finest spun glass. 



The -gV^h of Messrs. Powell and Lealand exhibited at King's 

 College, showed the Angulatum with woolly edges ; and doubtless 

 achromatism was obtained by some sacrifice of spherical correction. 

 The spherules were seen peeping as it were through a white mist in 

 which they appeared, as it were, partly dissolved. 



