650 Transactions of the Society. 



ture, of the well known kind, when examined with a Zeiss 24 mm. 

 (1 in.) apochromat of N.A. 0-32, axial illuminating cone of KA. 

 0*27 and X 12 ocular, to my eye practically only presented a very 

 beautiful and well contrasted wavy watered silk appearance, while 

 with the deeper eye-pieces the " exclamation " markings appeared 

 distinctly separated and their shape completely and well defined, 

 even the delicate sharply pointed ends being beautifully rendered 

 with the X 40 ocular, and the image of the scale thus magnified to 

 400 diameters appeared sharp and critical, without any signs of 

 undue strain on the defining qualities of the objective. 



Of course in both the above examples all the points mentioned 

 were in reality as fully resolved in the images afforded by the low 

 as in those obtained with the high eye-pieces, only in the former 

 instance the observer's eye proved incapable of grasping the minute 

 details owing to insufficient magnification. With such a result on 

 objects the structure of wliich was perfectly well known and 

 familiar, the necessity of judiciously employing sufficient enlarge- 

 ment when engaged in original research need hardly be further 

 insisted upon. The precise ratio of magnification to aperture 

 advisable will, however, undoubtedly depend on personal equation, 

 the nature of the object and the perfection of the objective and 

 adjustments. It is therefore impossible to lay down a hard-and- 

 fast rule on such a subject. 



The X 40 eye-piece referred to above was made for me by 

 Messrs. Powell and Lealand in 1901. It is compensated, but only 

 moderately, the over-correction being much less than in the Zeiss 

 oculars. In practice it has been found to work remarkably well, 

 not only with apochromats of all powers, but also with many old 

 achromatic objectives. For instance, I possess a Powell ^ in., 

 made in 1850, of measured N.A. 0-385. This lens, probably one 

 of the very finest old objectives ever made, when used with 

 Gifford's F-line screen, will stand the x 40 eye-piece and a full 

 solid illuminating cone without breaking down, and when thus 

 arranged exhibits a Ijrilliant, well contrasted picture of the minute 

 spines on the membrane of the blow-fly's proboscis. With the 

 screen and a solid 0*35 N.A. cone it will completely and cleanly 

 resolve Grayson's 30,000 band. The image of this is of course 

 faint, but by no means excessively so, and the performance of the 

 objective is astonishing when we consider that a Grayson's 30,000 

 band is very nearly the practical w^orking limit for a good modern 

 apochromat of similar aperture : indeed it is noted in my memo- 

 randa as extremely difficult with a fine Zeiss' 16 mm. apochromat 

 (really a ^ in.) of N.A. 0*35, an F-line screen and full solid illu- 

 minating cone. The glass of the old lens is to-day as clear and 

 good as when it first left the workshop, and it is gratifying to know 

 that an objective of such high perfection, even judged by modern 

 standards, was constructed over half a century ago by the famous 

 London firm. 



