ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 685 



bayonet catch, when it can be detached as required. By screwing 

 into the broad-gauge thread, after the binocular fitting is removed, 

 an adapter with Society screw, objectives of wide angle, but not 

 requiring the broad screw, can also be used without their being 

 handicapped by the fitting. 



This plan, by which there are in fact three distinct arrangements 

 — 1st, the broad-gauge screw ; 2nd, the Society screw without the 

 binocular : and 3rd, the Society screw with the binocular — is the one 

 adopted by Messrs. Sidle in the " Acme No. 2 " Microscope.* 



Homogeneous and Water-immersion Objectives.! — "Akakia," 

 replying to an inquiry whether homogeneous-immersion objectives are 

 to be regarded as useful only where apertures greater than the limit 

 for water (1 ■ 33 N.A.) are required, says that " experience has demon- 

 strated that all incident pencils from one refracting medium to another 

 of much greater refracting power, beyond the cone 140° in the rarer 

 medium, make unfavourable angles — angles that cannot be effectually 

 dealt with, and this applies the more the greater the difference 

 between the media. For instance, in a strictly dry lens the aperture 

 between the cone 140° in air (-94 N.A.) and 180° (1-0 N.A.) is prac- 

 tically of little use ; in a water-immersion lens the cone between 140° 

 in water (1 ■ 25 N.A.) and 180° (1 ■ 33 N.A.) is likewise of but little 

 service ; and equally in a homogeneous-immersion lens the cone 

 between 140° in the immersion fluid (1 ■ 43 N.A.) and 180° (1 ■ 52 N.A.) 

 is practically useless. Professor Abbe has arrived at the conclusion 

 that the limit of useful aperture is a much lower figure than 1 ■ 43 

 N.A. [Not so. He considers 1 ■ 45 the practical limit, ante, p. 472 — 

 Ed.] With our present means of construction, however, the lenses 

 which exhibit the finest definition with direct oblique illumination 

 that would utilize 1 ■ 25 N.A. are not those lenses of precisely 1 ■ 25 

 N.A., but of higher aperture. It would thus appear that in order to 

 get a well-corrected outer zone of 1*25 N A., the lens must really 

 have a larger aperture to cope successfully with the difficulties of the 

 marginal aberrations. It should be observed that by the homo- 

 geneous-immersion formula the higher apertures (say those beyond 

 • 94 N.A.) are more successfully corrected, because the path of the 

 rays is more regular, and can thus be more definitely calculated. 

 This is clearly evidenced by the superiority of definition seen with 

 homogeneous-immersion lenses, when, by the conditions of the object 

 and the illumination, the effective aperture is reduced well within the 

 limits that have already been attained by the water-immersion 

 formula : it is then seen that for all apertures greater than 1 • N.A. 

 the homogeneous-immersion formula is to be preferred. I believe it is 

 now generally accepted among expert manipulators that the water- 

 immersion formula has seen its best days, and the time is not far 

 distant when it will be entirely superseded." 



Collar Correction of Objectives.^ — Prof. A. Y. Moore considers 

 that collar correction has not received the attention which it deserves, 



* See also Bulloch's Congress Microscope, this Journal, iii. (1880) p. 1076, 

 t Engl. Mech, xxxv. (1882) p. 551. 

 % ' The Microscope,' ii. (1882) pp. 8-11. 

 Ser. 2.— Vol. II. 3 A 



