Page:Encyclopædia Britannica, Ninth Edition, v. 16.djvu/287

MICROSCOPE 271 view of the object is obtained, witb a gradational modification of the light. Another method, commonly adopted in German microscopes, is to place a draw-tube in the optic axis between the stage and the mirror, and to drop into the top of this tube one of a set of stops " perforated with apertures of different sizes ; this allows a gra dational effect to be obtained by raising or lowering the tube, so as to place the stop nearer to or more remote from the object ; but it is not nearly so convenient as the iris-diaphragm ; and the effect of the stop is not nearly so good when it is removed to some dis tance beneath the object as when it is very near to the under sur face of the glass object-slide. When an achromatic condenser is used, either a diaphragm-plate or an iris-diaphragm should be placed below its back lens, so as to cut off any required proportion of the outer rays that form its illuminating cone. Such an arrangement, while suiting all the ordinary requirements of the microscopist who uses the highest powers of his instrument for the purposes of biological investigation (as, for example, in the study of Bacteria or of the reproduction of the Monadina), does not serve to bring into effective use the special resolving power pos sessed by objectives of large aperture. It has long been known that for the discernment of very closely approximated markings oblique illumination is advantageous, an objective which exhibits such a diatom-valve as Pleurosigma angulatum with a smooth un marked surface when illuminated by the central rays of the achro matic condenser making its characteristic markings (figs. 8-11) distinctly visible when the central rays of the condenser are kept back by a stop, and the object is illuminated by its convergent mar ginal rays only. And it has also been practically known for some time that the resolution of lined or dotted tests can be often effected by nr rror illumination alone, if the mirror be so mounted as to be able to reflect rays through the object at such obliquity to the optic axis of the microscope as to reach the margin of a wide- angled objective. But it has only been since Professor Abbe s researches have given the true theory of "resolution" that the .special advantage of oblique illumination has been fully compre hended, and that the best means have been devised for using it effectively. Two different systems have now come into use, each of which has its special advantages. One consists in the attachment of the illuminating apparatus (mirror and achromatic condenser) to a "swinging tail-piece" (see fig. 32), which, moving radially upon a pivot whose axis intersects the optic axis at right angles in the plane of the object, can trans mit the illuminating pencil through it at any degree of obliquity that the construction of the stage allows. The direction of this pencil being of course limited to one azimuth, it is requisite, in order to bring out its full resolving effect, that the object should be made to rotate, by making the stage that carries it revolve round the optic axis, so that the oblique pencil may impinge upon the lines or other markings of the object in every direction successively. It will then bo found that the appearances presented by the same object often vary considerably, one set of lines being shown when the object lies in one azimuth, and another when its azimuth has been changed by rotation through 60, 90, or some other angle. Various contrivances have also been devised for throwing very oblique illuminating pencils on the object by means of prisms placed beneath the stage. Illumination of at least equal obliquity to that afforded by the swinging tail-piece may now, however, be obtained by the use of condensers specially constructed to give a divergence of 170 to the rays which they transmit when used immersionally, by bringing their flat tops into approximation to the under side of the glass slide on which the object is mounted, with the interposition of a film of water or (preferably) of glycerin. By using a central stop, marginal rays alone may be allowed to pass ; and these will be transmitted through the object in every azimuth at the same time. But diaphragms with apertures limiting the transmitted rays to one part of the periphery may be so fixed in a tube beneath the condenser as to be easily made to rotate, thus sending its oblique pencils through the object in every azimuth in succession. And where this rotation of the diaphragm brings out two sets of lines at a certain angular interval a diaphragm with two marginal openings at a corresponding angular distance will enable both to be seen at once. Numerous arrangements of this kind have been devised by those who devote their special attention to the reso lution of difficult diatom-tests ; but they are of little or no use to those who use the microscope for biological research. For the illumination of the surfaces of opaque objects which must be seen by reflected light the means employed will vary with the focal length of the objective employed. For large bright objects viewed under a low magnifying power good ordinary daylight is sufficient ; but if the surface of the object is dull, reflecting but little light, the aid of a bull s-eye or large bi-convex lens must be employed in order to give it sufficient brilliance. This aid will always be required by lamplight ; and by a proper adjustment of the relative distances of the lamp and the object the rays from the lamp may be made either to spread themselves over a wide area or to converge upon a small spot. The former is the method suitable to large objects viewed under a low magnifying power ; the latter to the illumination of small objects which are to be examined under objectives of (say) 1 inch or | inch focus. Another method which may be conveniently had recourse to when the microscope is pro vided with a swinging tail-piece is to turn this on its pivot until the concave mirror is brought above the stage, so that rays which it gathers cither from natural or artificial sources may be reflected downwards upon the surface of the object. The illumination of an opaque object to be seen with a higher power than the f or ^ inch objectives was formerly provided for by a concave speculum (termed a Lieberkiihn after its inventor), with a perforation in the centre for the passage of the rays to the objec tive to which it is fitted, the curvature of the speculum being so adapted to the focus of the objective which carries it that, when the latter is duly adjusted, the rays reflected upwards around the object from the mirror to the speculum shall converge strongly on the ob ject. The various disadvantages of this mode of illumination, however, have caused it to be now generally superseded by other arrangements. For powers between 1^ inch and -fa inch, and even for a or inch of small angle and good working distance, nothing is so convenient as the parabolic speculum or side- illuminator (F, fig. 17) invented by the late Richard Beck. This is attached to a spring-clip that slides on the tubes of low-power objec tives, so that its distance from the object and the direction of its re flected pencil are readily adjusted ; FIG. 17. Beck s Parabolic Side- and for use with higher powers it may be either mounted on a separ with Crouch s Illuminator, Adapter. ate" arm attached to some part of the stand of the microscope, or may be hung in the manner shown in fig. 17 from an " adapter " A interposed between the objective and the body. By rotating the collar B and making use of the joints C, C, the lengthening rod D, and the ball and socket K, any position may be given to the speculum F that may best suit the objective with which it is used. When, however, it is desired to illuminate objects to be seen under objectives of high power and very short working distance, side illumination of any kind becomes difficult, though not absolutely impossible ; l and various modes have been devised for the illumina tion of the object by means of light sent down upon it, through the objective, from above. This is done in the vertical illuminator of Messrs Beck (fig. 18) the original idea of which was first A C FIG. 18. Beck s Vertical Illuminator. given by the American Professor II. L. Smith by a disk of thin glass B, b, attached to a milled head by which its angular position may be adjusted, and introduced by a slot A, e into the interior of an adapter that is interposed between the objective C, d and the nose c of the body. The light which enters at the lateral aperture A, a, falling upon the oblique surface of the disk C, b, is reflected downwards, and is concentrated by the lenses of the objective upon the object beneath. The lateral aperture may be provided with a diaphragm, with openings of different sizes, for diminishing the false light to which this method is liable ; or a screen with a small aperture may be placed between the lamp and the l See a method devised ty Mr James Smith, in Jour. Roy. Micros. Soc., vol. iii.,

N. S., 1880, p. 398.