(D) ILLUMINATING APPARATUS
Every improvement in the optical performance of the compound achromatic microscope has called forth a corresponding improve-ment in the illumination of the objects viewed by it, since it soon came to be apparent that without such improvement the full advantage of the increased defining and resolving powers of the objectives could not be obtained. For the illumination of trans-parent objects examined by light transmitted through them under low powers of moderate angle a converging pencil of rays reflected upon their under surface by a concave mirror is generally sufficient,a "condenser" being only needed when the imperfect transparency of the object requires the transmission of more light through it. And the microscopist engaged in ordinary biological studies, who, works on very transparent objects with objectives of 1/6 or 1/8 inch focus, or 1/6 inch immersion, will find that the small concave mirror of short focus with which the Continental models are furnished (see fig. 28) will generally prove sufficient for his needs. This mirror is usually hung at such a distance beneath the stage that parallel rays falling on it are brought to a focus in the object as it lies on a slip of glass resting on the stage; and thus, when the instrument is used by day, the light of a bright cloud (which is pre-ferable to any other) gives a well-illuminated field, even with the powers last-mentioned. But when lamplight is used its divergent rays are not brought to a focus in the object by a mirror that is, fixed as just stated; and the distance of the mirror beneath the stage should be made capable of increase (which is easily done by attaching it to a lengthening bar), so as to obtain the requisite focal convergence. Still the best effects of objectives of less than 1/4 inch focus cannot be secured without the aid of an achromatic condenser, interposed between the mirror and the object, so as to bring a larger body of rays to a more exact convergence.
When objectives of still higher power are employed, the employ-ment of such a condenser becomes indispensable; and when the highest powers are being used by lamplight, it is desirable to dis-pense with the mirror altogether, and to place the flame exactly in the optic axis of the microscope. The condenser should be an achromatic combination, corrected for the ordinary thickness of the glass slip on which the object lies, and capable of being so adjusted as to focus the illuminating pencil in the object.
As it is often found desirable that an object should be illuminated by central rays alone, or that the quantity of light transmitted through it should be reduced (for bringing into view delicate details of structure which are invisible when the object is flooded with light), every microscope should be provided with some means of cutting off the outer rays of the illuminating cone. The "dia-phragm-plate" ordinarily used for this purpose is a disk of black metal, pivoted to the under side of the stage, and perforated with a graduated series of apertures of different diameters, any one of which can be brought, by the rotation of the disk, exactly into the optic axis of the microscope. But the required effect can be much more advantageously obtained by the "iris-diaphragm," in which a number of converging plates of metal are made so to slide over each other by the motion of a lever or screw that the aper-ture is either enlarged or diminished, while always remaining prac-tically circular as well as central; and in this manner a continuous view of the object is obtained, with 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 gradational 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 distance beneath the object as when it is very near to the under surface 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 possessed 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 unmarked surface when illuminated by the central rays of the achromatic 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 marginal 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 mirror 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 Abbes researches have given the true theory of "resolution" that the special advantage of oblique illumination has been fully comprehended, 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 transmit 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 be 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 60º, 90º, 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 170º 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 opening 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 resolution 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 object 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 bulls-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 2/3 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 either 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 2/3 or 1/2 inch objectives was formerly provided for by a concave speculum (termed a Lieberkühn after its inventor), with a perforation in the centre for the passage of the rays to the objective 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 object. The various disadvantages of this mode of illumination, however, have caused it to be now generally superseded by other arrangements. For powers between 1 1/2 inch and 4/10 inch, and even for a 1/4 or 1/5 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 objectives, so that its distance from the object and the direction of its reflected pencil are readily adjusted; and for use with higher powers it may be either mounted on a separ-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 interpose 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 E, 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; [Footnote 271-1] and various modes have been devised for the illumination 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 given by the American Professor H. L. Smithby 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 illuminator, at any distance that is found to produce the best effects. In using this illuminator, the lamp should be placed at a distance of about 8 inches from the aperture; and, when the proper adjustments have been made, the image of the flame should be seen upon the object. The illumination of the entire field, or the direction of the light more or less to either side of it, can easily be managed by the interposition of a small condensing lens placed at about the distance of its own focus from the, lamp. The objects viewed by this mode of illumination with dry-front objectives are best uncovered, since, if they are covered with thin glass, so large a proportion of the light sent down upon them is reflected from the cover (especially when objectives of large angle of aperture are employed) that very little is seen of the objects beneath, unless their reflective power is very high. With immersion objectives, however, covered objects may be used. Another method of vertical illumination long since devised by Mr Tolles has recently been brought into notice by Professor W. A. Rogers of Boston (U. S.). It consists in the introduction of a small rectangular prism at a short distance behind the front combination of the objective, so that parallel rays entering its vertical surface pass on between its parallel horizontal surfaces until they meet the inclined surface, by which they are reflected downwards. In passing through the front combination of the objective, they are deflected towards its axis; but, as their angle of convergence is less than the angle of divergence of the rays proceeding from the object, the reflected rays will not meet in the focal point of the lens, but will be so distributed as to illuminate a sufficient area. By altering the extent to which the prism is pushed in, or by lifting or depressing its outer end by means of a milled-head screw, the field of illumination can be regulated. The working of this with immersion-objectives is stated by Mr Tolles to be peculiarly satisfactory.
271-1 See a method devised by Mr James Smith, in Jour. Roy. Micros. Soc., vol. iii., N. S., 1880, p. 398.
Read the rest of this article:
Microscope - Table of Contents