84. Although the most perfect correction required for each particular object (which depends, not merely upon the thickness of its glass cover, but upon that of the fluid or balsam in which it may be mounted) can only be found by experimental trial, yet for all ordinary purposes, the following simple method, first devised by Mr. Powell, will suffice. The object-glass, adjusted to "uncovered," is to be "focussed" to the object; its screw-collar is next to be turned, until the surface of the glass cover comes into focus, as may be perceived by the spots or striæ by which it may be marked; the object is then to be again brought into focus by the "fine movement." The edge of the screw-collar being now usually graduated, the particular adjustment which any object may have been found to require, and of which a record has been kept, may be made again without any difficulty. By Messrs. Smith and Beck, however, who first introduced this graduation, a further use is made of it. By experiments such as those described in the last paragraph, the correct adjustment is first found for any particular object, and the number of divisions observed, through which the screw-collar must be moved in order to bring it back to 0°, the position suitable for an uncovered object. The thickness of the glass cover must then be measured by means of the "fine movement:" this is done by bringing into exact focus, first the object itself, and then the surface of the glass cover, and by observing the number of divisions through which the milled head (which is itself graduated) has passed in making this change. A definite ratio between that thickness of glass, and the correction required in that particular objective, is thus established; and this serves as the guide to the requisite correction for any other thickness, which has been determined in like manner by the "fine movement." Thus, supposing a particular thickness of glass to be measured by 12 divisions of the milled head of the fine movement, and the most perfect performance of the object-glass to be obtained by moving the screwcollar through 8 divisions, then a thickness of glass measured by 9 divisions of the milled head, would require the screw-collar to be adjusted to 6 divisions in order to obtain the best effect. The ratio between the two sets of divisions is by no means the same for different combinations; and it ought to be determined for each objective by its maker, who will generally be the best judge of the best "points" of his lenses; but when this ratio has been once ascertained, the adjustment for any thickness of glass with which the object may happen to be covered, is readily made by the Microscopist himself. Although this method appears somewhat more complex than that of Mr. Powell, yet it is more perfect and when the ratio between the two sets of divisions has been once determined, the adjustment does not really involve

ment is thus performed within a third part of a revolution, with scarcely any friction, and with such an immediate transition from good to bad definition, that the best point is made readily apparent.

more trouble. Another use is made of this adjustment by Messrs. Smith and Beck; namely, to correct the performance of the objectives, which is disturbed by the increase of distance between the objective and the eye-piece, that is occasioned by the use of the draw-tube (§ 43). Accordingly, they mark a scale of inches on the draw-tube (which is useful for many other purposes), and direct that for every inch the body is lengthened, the screw-collar of the objective shall be moved through a certain number of divisions.

85. Arrangement for Transparent Objects.-If the object be already "mounted" in a slide, nothing more is necessary, in order to bring it into the right position for viewing it, than to lay the slide upon the object-platform of the stage, and to support it in such a position (by means of the sliding ledge or other contrivance) that the part to be viewed is, as nearly as can be guessed, in the centre of the aperture of the stage, and therefore in a line with the axis of the body. If the object be not "mounted," and be of such a kind that it is best seen dry, it may be simply laid upon the glass stage-plate (§ 67), the ledge of which will prevent it from slipping off when the microscope is inclined, and a plate of thin glass may be laid over it for its protection, if its delicacy should seem to render this desirable. If, again, it be disposed to curl up, so that a slight pressure is needed to flatten or extend it, recourse may be had to the use of the aquatic box (§ 68), or of the compressorium (§ 70), no liquid, however, being introduced between the surfaces of glass. In a very large proportion of cases, however, either the objects to be examined are already floating in fluid, or it is preferable to examine them in fluid, on account of the greater distinctness with which they may be seen; if such objects be minute, and the quantity of liquid be small, the drop is simply to be laid on a slip of glass, and covered with a plate of thin glass; if the object or the quantity of liquid be larger, it will be better to place it in the aquatic box; whilst, if the objects have dimensions which render even this inconvenient, the zoophyte trough (§ 69) will afford the best medium for its examination. If it be wished to have recourse to compression, for the expansion or flattening of the object, this may be made upon the ordinary slide, by pressing down the thin glass cover with a pointed stick; and this method, which allows the pressure to be applied where it may chance to be most required, will generally be found preferable for delicate portions of tissue which are easily spread out, and which, in fact, require little other compression than is afforded by the weight of the glass cover, and by the capillary attraction which draws it into proximity with the slide beneath. A firmer and more enduring pressure may be exerted by the dexterous management of a well-constructed aquatic box; and this method is peculiarly valuable for confining the movements of minute animals, so as to keep them at rest under

the field of the microscope, without killing them. It is where a firm but graduated pressure is required, for the flattening out of the bodies of thin semi-transparent animals, without the necessity of removing them from the field of the microscope, that the compressorium is most useful. Wherever the first and simplest of the above methods can be had recourse to, it is the preferable one; since the object, when on a glass slide, can be subjected to the Achromatic Condenser, Polariscope, Oblique Illumination, &c., with far more convenience than when removed to a plane above the stage, as it must be when the aquatic box is used. Whether the object be submitted to examination on a slip of glass, or in the aquatic box or compressorium, it must be first brought approximately into position, and supported there, just as if it were in a mounted slide. The precise mode of effecting this will differ, according to the particular plan of the instrument employed; thus in some, it is only the ledge itself that slides along the stage; in others, it is a carriage of some kind, whereon the object-slide rests; in others, again, it is the entire platform itself that moves upon a fixed plate beneath.

86. Having guided his object, as nearly as he can do by the unassisted eye, into its proper place, the Microscopist then brings his light (whether natural or artificial) to bear upon it, by turning the mirror in such a direction as to reflect upon its under surface the rays which are received by itself from the sky or the lamp. The concave mirror is that which should always be first employed, the plane being reserved for special purposes; and it should bring the rays to convergence in or near the plane in which the object

lies (Fig. 54). The distance at which it should be ordinarily set beneath the stage, is that at which it brings parallel rays to a focus; but this distance should be capable of elongation, by the lengthening of the stem to which the mirror is attached; since the rays diverging from a lamp at a short distance, are not so soon brought to a focus. The correct focal adjustment of

FIG. 54.

Arrangement of Microscope for Transparent Objects.

the mirror may be judged of, by its formation of images of window-bars, chimneys, &c., upon any semi-transparent medium placed in the plane of the object. It is only, however, when

small objects are being viewed under high magnifying powers, that such a concentration of the light reflected by the mirror is either necessary or desirable; for with large objects, seen under low powers, the field would not in this mode be equably illuminated. The diffusion of the light over a larger area may be secured, either by shifting the mirror so much above or so much below its previous position, that the pencil will fall upon the object whilst still converging or after it has met and diverged; or, on the other hand, by the interposition of a plate of ground glass in the course of the converging pencil,-this last method, which is peculiarly appropriate to lamp-light, being very easily had recourse to, if the diaphragm plate, as formerly recommended (§ 55), have had its largest aperture filled with such a diffused medium. The eye being now applied to the Eye-piece, and the body being "focussed," the object is to be brought into the exact position required, by the use of the traversing movement, if the stage be provided with it; if not, by the use of the two hands, one moving the object-slide from side to side, the other pushing the ledge, fork, or holder that carries it, either forwards or backwards, as may be required. It is always to be remembered, in making such adjustments by the direct use of the hands, that, owing to the inverting action of the microscope, the motion to be given to the object, whether lateral or vertical, must be precisely opposed to that which its image seems to require, save when the Erector (§ 44) is employed. When the object has been thus brought fully into view, the Mirror may require a more accurate adjustment. What should be aimed at, is the diffusion of a clear and equable light over the entire field; and the observer should not be satisfied, until he has attained this object. If the field should be darker on the one side than on the other, the mirror should be slightly turned in such a direction as to throw more light upon that side; perhaps in so doing, the light may be withdrawn from some part previously illuminated; and it may thus be found that the pencil is not large enough to light up the entire field. This may be owing to one of three causes: either the cone of rays may be received by the object too near to its focal apex, the remedy for which lies in an alteration in the distance of the mirror from the stage; or, from the very oblique position of the mirror, the cone is too much narrowed across one of its diameters, and the remedy must be sought in a change in the position either of the microscope or of the lamp, so that the face of the mirror may not be turned so much away from the axis of vision; or, again, from the centre of the mirror being out of the optical axis of the instrument, the illuminating cone is projected obliquely, an error which can be rectified without the least difficulty. If the cone of rays should come to a focus in the object, the field is not unlikely to be crossed by the images of window-bars or chimneys, or the form of the lamp-flame may be

distinguished upon it; the former must be got rid of by a slight change in the inclination of the mirror; and if the latter cannot be dissipated in the same way, the lamp should be brought a little nearer.

87. The equable illumination of the entire field having been thus obtained, the quantity of light to be admitted should be regulated by the Diaphragm-plate (§ 55). This must depend very much upon the nature of the object, and upon the intensity of the light. Generally speaking, the more transparent the object, the less light does it need for its most perfect display; and its most delicate markings are frequently only made visible, when the major part of the cone of rays has been cut off. Thus the movement of the cilia,-those minute vibratile filaments, with which almost every Animal is provided in some part of its organism, and which many of the humbler Plants also possess,— can only be discerned in many instances, when the light is admitted through the smallest aperture. On the other hand, the less transparent objects usually require the stronger illumination which is afforded by a wider cone of rays; and there are some (such as semi-transparent sections of fossil teeth) which, even when viewed with low powers, are better seen with the intenser light afforded by the Achromatic Condenser. In every case in which the object presents any considerable obstruction to the passage of the rays through it, great care should be taken to protect it entirely from incident light; since this extremity weakens the effect of that which is received into the microscope by transmission. It is by daylight that this interference is most likely to occur: since, if the precautions already given (§ 76) respecting the use of lamp-light be observed, no great amount of light can fall upon the upper surface of the object. The observer will be warned that such an effect is being produced, by perceiving that there is a want, not only of brightness, but of clearness, in the image; the field being veiled, as it were, by a kind of thin vapor; and he may at once satisfy himself of the cause, by interposing his hand between the stage and the source of light, when the immediate increase of brilliancy and of distinctness will reveal to him the occasion of the previous deficiency in both. Nothing more is necessary for its permanent avoidance, than the interposition of an opaque screen (blackened on the side towards the stage) between the window and the object; care being of course taken, that the screen does not interfere with the passage of light to the mirror. Such a screen may be easily shaped and adapted either to be carried by the stage itself, or by the stand for the condenser; but it is seldom employed by Microscopists, as it interferes with access to the left side of the stage; and the interposition of the hand, so often as it may be needed, is more frequently had recourse to in preference, as the more convenient expedient. The young Microscopist who may be examining transparent objects by daylight, is recommended never to omit