Dec. 1, 1865.] 



SCIENCE-GOSSIP. 



269 



contact with a horse-shoe maguet attached to the 

 luider-surface of the stage, are rendered magnetic by 

 it. A soft iron bar resting upon them at right angles, 

 serves as a ledge for objects to rest against when the 

 microscope is inclined from the vertical position. 

 The attraction between the bar and the tongues is 

 just suihcient to keep the former in its place witli 

 any ordinary object that may be resting against it, 

 and is not too strong for being easily shifted about. 



As the modern microscope possesses many adjuncts 

 —as condensers, spotted lenses, polarizers, &c., allot 

 which are used beneath the stage ; and as these need 

 permanent support and accurate centering, a move- 

 able sub-stage is constructed, into which they all fit. 

 A traversing movement is given to the stage itself, 

 by rack-work and pinion. The microscopist is thus 

 enabled to shift the position of his various apparatus 

 to suit his requirements. Between the stage and 

 sub-stage is a shutter for regulating the amount of 

 light reflected upward from the mirror; it is termed 

 the diaphragm, and consists of a thin circular plate 

 perforated with holes varying in diameter, the largest 

 being equal to the central aperture of the stage, and 

 the smallest not much larger than a pin-hole, while 

 in one position the light is cut off altogether. 

 The plate revolves on a pivot, so that each aperture 

 may, in its turn, be brought under the stage. The 

 diaphragm should never be permanently fixed to the 

 stage, but should be so attached as to be easily 

 i-emoved when other modes of illumination are 

 desired. An improved form of diaphragm has 

 lately been constructed, in which the circular 

 shutter with its various openings has been super- 

 seded, and in its stead we have but one aperture, 

 which can be gradually closed by two small plates 

 that move horizontally to the right and left of the 

 stage, and have their proximal borders incised in a \ 

 wedge-shaped manner. The advantage of this ' 

 diaphragm consists in the nicety with which the ' 

 orifice can be regulated so as to obtain the most 

 effective illumination. 



Another important part of the microscope is 

 the mirror, by whose aid we collect and reflect the 

 light upwards to the object-glass. This should 

 in all cases be double, having one of its surfaces 

 flat and the other concave, with a diameter not less 

 than an inch and a half, or even more. The greater 

 the extent of the reflecting surface, the larger 

 will be the amount of light obtained therefrom. 

 As so much depends on the proper illumination of 

 an object, the mirror should be so constructed as to 

 admit of free and easy motion in all directions. It 

 sliould slide freely up and down the stem which 

 supports it, thereby enabling the microscopist to 

 regulate the intensity of its light as he increases or 

 decreases its distance from the stage ; and if it be 

 mounted on a lengthening arm, a further advantage 

 v\dll be gamed by the oblique illumination it pro- 

 duces. 



When high powers are used, a more intense light 

 than that afforded by the mirror is needed. The 

 microscopist then has recourse to the various kinds 

 of condensers in vogue. These are usually achro- 

 matic combinations of lenses, and, when properly 

 adjusted, condense on the object in one brilliant spot 

 all the light reflected fi'om the mirror. Our space, 

 however, precludes us from entering further into the 

 subject of illumination. 



Having selected a good solid stand, the micro- 

 scopist should next secure the best lenses that can be 

 procured from our most skilful opticians. These may 

 be purchased one at a time, according to the means 

 and wants of the purchaser. Two of the most useful 

 powers are the one-inch and one-fourth objective ; 

 and with these two powers a large amount of useful 

 work may be done. Good object-glasses may be 

 known by the clear and distinct images they give of 

 suitable objects placed in their foci, and a bad ob- 

 jective may be equally well known by the absence of 

 these qualities. No amount of careful focussing will 

 ever enable such a lens to give a clear and well- 

 defined view of an object submitted to its scrutiny. 

 The eye of the observer, however keen, sees nothing 

 distinctly where all is involved in fog. 



One very good test of the quality of an object-glass 

 of moderate power is the way in which it bears the 

 strain of the higher eye-pieces, even when the draw 

 tube is pulled out to some extent. If the combina- 

 tion be properly corrected for light and colour, it 

 will give a flat field, with clear definition in every 

 part of it, and will suffer scarcely any deterioration 

 in its performance. The reverse of this will hold 

 good with inferior lenses. The question of angular 

 aperture scarcely comes within the scope of a short 

 paper ; but those who are interested in this branch of 

 optics will find it fully discussed in our standard 

 T/orks on the microscope. The ordinary method of 

 attaching an object-glass to the body of the micro- 

 scope is by screwing it into the tube, and as the 

 threads of the screw are generally very fine, some little 

 time is consumed in substituting one lens for another; 

 to obviate this difiiculty, and to facilitate a rapid 

 exchange of objectives, a mechanical contrivance has 

 been invented, termed a nose-piece, which consists 

 of two or more rotating arms, each carrying an ob- 

 jective at its free end. By the rotation of the arms, 

 each lens, in its turn, is brought into its proper posi- 

 tion beneath the tube. 



The eye-pieces (of which some three or four with 

 varying magnifying pov\'ers are supplied by our best 

 makers) should be constructed so as to slide gently 

 into the upper end of the body, a mode of attach- 

 ment that gives great facihty to the microscopist for 

 changing his eye-pieces. 



One of the greatest modern improvements in con- 

 nection with the compound microscope undoubtedly 

 is Mr. Wenham's binocular arrangement. Its sim- 

 plicity and facility of adaptation to almost every kind 



