1889.] MICROSCOPICAL JOURNAL. 59 



Pleurosigma into beads, bearing the half-inch solid eye-piece to per- 

 fection, and with this objective, under these conditions, the bright spot 

 may be seen, when the eye-piece is removed, to sweep around the edge 

 of the back lens like a drop of fire. To accomplish this with the 

 condenser of 1.40. N. A., it and the objective must be in immersion 

 contact with the slide, while the objective itself should be a wide- 

 angled homogeneous immersion. 



If the substage has lateral movements, as it has on some first-class 

 stands, oblique illumination with the circular diaphragm openings may 

 be obtained, but somewhat less effectually, by moving the entire con- 

 denser from side to side. 



For black-ground illumination, the central disk diaphragm must be 

 used, and with the one-inch objective brilliant effects may be produced, 

 under this illumination, with the proper objects. To do this with the 

 one-inch of 33 and the condenser of 1.40 N. A., it is necessary to re- 

 move the front lens of the condenser, when the effect will be exceed- 

 ingly fine. Here again if the bull's-eye lens is placed between the 

 mirror and the source of light, the plane mirror is to be used ; if the 

 light is taken directly from the lamp flame, the concave mirror is the 

 proper one to be employed. Black-ground illumination may be ob- 

 tained with powers of from five hundred to six hundred diameters, but 

 according to my experience it is not praiseworthy. The field is not 

 brilliant, the object does not glow with that peculiar and atti-active 

 silvery fire that seems to come from an internal source, but the picture 

 is foggy with a bluish mist, and the silvery gleams are dull and lifeless. 



With his form of the condenser, Dr. Zeiss supplies diaphragms to be 

 applied to the back lens of the wide-angled objectives when black-ground 

 illumination is desired with them. The proper disk-bearing plate is 

 placed in the diaphragm carrier, and immersion contact is made with 

 the lower surface of the slide. The difficulty with any but Zeiss's 

 objectives is to prepare the diaphragms for the back lens. With his 

 they may be made of metal, to properly fit when dropped into the 

 mounting, and the opening will then be central. But if the micros- 

 copist must cut them from paper, he need not expect to obtain the best 

 results. The directions are to drop diaphragms into the back of the 

 wide-angled objectives, and then the microscopist is left, by all except 

 Zeiss, to take care of himself. Yet it is, of course, impossible for any 

 one optician to supply these little parts, since no two objectives of even 

 the same magnifying power have the same sized mounting. The micros- 

 copist must depend upon himself, and he will speedily observe that 

 when the aperture is reduced by a diaphragm at the back lens, the de- 

 fining and resolving powers of the objective suffer an injurious diminu- 

 tion. The experiment is worth making though no other result than 

 this be obtained. 



While black-ground illumination is beautiful, it has little scientific 

 value. I do not know that any discovery, or even any observation of 

 importance, has ever been made by its use. It will at times exhibit 

 certain structural features in a conspicuous way, but only, I think, after 

 they have been previously observed, for in most cases this peculiar 

 lighting appears to make the structure obscure. It may render the 

 contour lines more distinct, and envelop the whole object in a glamour 

 of brilliant beauty, but the microscopist, while he never disdains beauty ,_ 

 never makes it the object of his pursuit. 



