ascertaining, whether the view which he may obtain of them, is in any degree thus marred by incident light.

88. Although the illumination afforded by the mirror alone is quite adequate for a very large proportion of the purposes for which the Microscope may be profitably employed (nothing else having been used by many of those who have made most valuable contributions to Science by means of this instrument), yet, when high magnifying powers are employed, and sometimes even when but a very moderate amplification is needed, great advantage is gained from the use of the Achromatic Condenser. The various modes in which this may be constructed, and may be fitted to the Microscope, have been already described (§ 56); we have now to speak of the manner of using it. The lenses with which the Condenser is provided should be made to separate from each other, in such a manner that two or three distinct powers should be afforded; the complete combination should be used with objectives of 1-5th inch focus or less; the front lens should be removed with objectives of from half to a quarter of an inch focus; and the second lens may be removed, so that the back lens will be alone employed, when it is desired to use the condenser with objectives of less than half an inch focus. It is of the greatest importance that the Condenser should be accurately adjusted, both as to the coincidence of its optical axis with that of the Microscope itself, and as to its focal distance from the object. The centring may be most readily accomplished, by so adjusting the distance of the condenser from the stage (by the rack-and-pinion action, or the sliding movement, with which it is always provided), that a sharp circle of light shall be thrown on any semi-transparent medium laid upon it; then, on this being viewed through the Microscope with an objective of sufficiently low power to take in the whole of it, if this circle be not found to be concentric with the field of view, the axis of the condenser must be altered by means of the milledhead tangent screws with which it is provided. The focal adjustment, on the other hand, must be made under the objective which is to be employed in the examination of the object, by turning the mirror in such a manner as to throw upon the visual image of the object (previously brought into the focus of the Microscope) an image of a chimney or window-bar, if daylight be employed, or of the top, bottom, or edge of the lamp-flame, if lamplight be in use; such a vertical movement should be given to the condenser, as may render the view of this as distinct as possible; and the direction of the mirror should then be sufficiently changed to displace these images, and to substitute for them the clearest light that can be obtained. It will generally be found, however, that although such an exact focussing gives the most perfect results by daylight, yet that by lamp-light the best illumination is obtained, when the condenser is removed to a somewhat greater distance from the object, than that at which it gives

a distinct image of the lamp. In every case, indeed, in which it is desired to ascertain the effect of variety in the method of illumination, the effects of alterations in the distance of the condenser from the object should be tried; as it will often happen that delicate markings become visible when the condenser is a little out of focus, which cannot be distinguished when it is precisely in focus. The diaphragm-plate with which all the best forms of Achromatic Condenser are now furnished, enables the observer not only to vary the angle of his illuminating pencils through a range of from 20° to 80°, but also to stop off the central portion of the pencil, so as to allow only its most oblique rays to pass; and the contrast presented by the aspect of many objects, according as the size and form of the aperture in the diaphragm-plate limits the rays transmitted by the condenser to those of the central or those of the peripheral portion of the pencil, is often so marked, as to show beyond question the great importance of this mode of varying the illumination. When the condenser is employed, the plane Mirror may often be substituted with advantage for the concave; the chief effect of this exchange being to diminish the quantity of light, without altering the angle of the illuminating pencil. It must be borne in mind, in making such an alteration, that the plane mirror reflects parallel or (if from a lamp) diverging rays, instead of the converging rays reflected by the concave mirror; so that the focus of the condenser is likely to require readjustment. For objects of great delicacy and transparency, the "white-cloud" illumination (§ 58) may be had recourse to with advantage; or, if it be desired that the illuminating pencil should be free from the error imparted by the double reflection of the mirror, the mirror may be turned aside, and in its stead the lamp (if the observation be made by artificial light) may be placed in the axis of the microscope; or the mirror may be replaced by "Dujardin's prism" (§ 57), which will be equally available either by daylight or by lamp-light.

89. Should it be desired, however, to try the effect of very oblique light upon an object, the Achromatic Condenser must be removed (unless, as in Mr. Sollitt's arrangement, § 130, it be so constructed as to be capable of inclination to the axis of the Microscope), and other means must be employed. The simplest method, where the mirror is mounted on an "arm" (Fig. 29), is to turn it to one side so as to reflect the rays at a considerable angle; and where this cannot be done, nearly the same effect is produced by placing the lamp in the direction from which it is desired that the oblique rays should proceed, and interposing an ordinary condensing lens between it and the object. Or, if the Microscopist be provided with the means of mounting a "Dujardin's prism" on a separate stand, he may place it in such a position as to reflect light from any point required: and he may concentrate that light by an ordinary condenser. The possession

of Amici's prism, however (which serves both as mirror and condenser, § 60), will save the necessity of any other provision of this kind. It is when objects are thus illuminated by oblique light, and when their markings are of such a kind as to be best or to be only shown by light falling upon them in one particular direction, that we derive the greatest advantage from the power of giving a rotatory movement either to the object or to the illu

C

Α

B

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minating apparatus. Thus suppose that an object be marked by longitudinal striæ, too faint to be seen by ordinary direct light; the oblique light most fitted to bring them into view, will be that proceeding in either of the directions c or D; that which falls upon it in the directions A and B, tending to obscure the striæ rather than to disclose them. But, moreover, if the striæ should be due to furrows or prominences which have one side inclined and the other side abrupt, they will not be brought into view indifferently by light from c or from D, but will be shown best by that which makes the strongest shadow; hence if there be a projecting ridge, with an abrupt side looking towards c, it will be best seen by light from D; whilst if there be a furrow with a steep bank on the side of c, it will be by light from that side that it will be best displayed. But it is not at all unfrequent for the longitudinal striæ to be crossed by others; and these transverse striæ will usually be best seen by the light that is least favorable for the longitudinal; so that, in order to bring them into distinct view, either the illuminating pencil or the object must be moved a quarter round. The revolving action with which the stage of Mr. Ross's Microscope is provided (§ 37), enables this movement to be given to the object without any displacement of its image, which, of course, executes, to the eye of the observer, a rotation in the opposite direction. In other microscopes, however, it is difficult to give a rotation to the object, by causing the object-platform to turn upon its axis, without throwing the object out of the field (§ 38); though this may be accomplished, by such an adjustment of the traversing movement, as shall bring the centre of the tube on which that platform turns round, into the visual axis of the microscope-or, if this adjustment cannot be conveniently made in the first instance, by keeping the right hand constantly in action upon the milled heads of the stage movement, whilst the left hand rotates the object-platform, so as, by means of the former, to correct the displacement of the object occasioned by the latter. It may be sufficient, however, to examine the object in several different positions, so that the appearances it presents in each may be compared, without thus watching the transition from one to the other.

90. There are many kinds of transparent objects, especially such as either consist of thin plates, disks, or spicules of siliceous or calcareous matter, or contain such bodies, which are peculiarly

well seen under the black-ground illumination (§§ 61, 62); for not only does the brilliant luminosity which they then present, contrasting remarkably well with the dark ground behind them, show their forms to extraordinary advantage; but this mode of illumination imparts to them an appearance of solidity, which they do not exhibit by ordinary transmitted light (§ 62); and it also frequently brings out surface-markings, which are not otherwise distinguishable. Hence, when any object is under examination, that can be supposed to be a good subject for this method, the trial of it should never be omitted. For the low powers, the use of the "spotted lens" will be found sufficiently satisfactory; for the higher, the paraboloid should be employed (§ 61). Similar general remarks may be made, respecting the examination of objects by polarized light. Some of the most striking effects of this kind of illumination, are produced upon bodies whose particles have a crystalline aggregation; and hence it may often be employed with great advantage to bring such bodies into view, when they would not otherwise be distinguished; thus, for example, the raphides of Plants are much more clearly made out by its means, in the midst of the vegetable tissues, than they can be by any other. But the peculiar effects of polarized light are also exerted upon a great number of other organized substances, both Animal and Vegetable; and it often reveals differences in the arrangement or in the relative density of their component particles, the existence of which would not otherwise have been suspected; hence, the Microscopist will do well to have recourse to it, whenever he may have the least suspicion that its use can give him an additional power of discrimination.

91. Arrangement for Opaque Objects. Although a large proportion of the objects best suited for Microscopic examination are either in themselves sufficiently transparent to admit of being viewed by light transmitted through them, or may be made so by appropriate means, and although that method (where it can be adopted) is generally the one best fitted for the elucidation of the details of their structure, yet there are many objects of the most interesting character, the opacity of which entirely forbids the use of this method, and of which, therefore, the surfaces only can be viewed, by means of the incident rays which they reflect. These are, for the most part, objects of comparatively large dimensions, for which a low magnifying power suffices; and it is specially important, in the examination of such objects, not to use a lens of shorter focus than is absolutely necessary for discerning the details of the structure; since, the longer the focus of the objective employed, the less is the indistinctness produced by inequalities of the surface, and the larger, too, may be its aperture, so as to admit a greater quantity of light, to the great improvement of the brightness of the image. It is surprising how little attention has been given by Opticians to the construction of objectives suitable for this purpose. In their zeal for the im

provement of the higher powers of the Microscope, they have thought comparatively little of the lower; and in Continental Microscopes, it is rare to meet with an objective which will give even a tolerable view of a large opaque object. The Author, indeed, well remembers the time, when it was not thought worth while, even by English Opticians, to construct Achromatic object-glasses of less than an inch focus; and the production of objectives of 13 inch and 2 inch focus has been chiefly called for, in consequence of their value in displaying anatomical preparations in which the bloodvessels have been injected with coloring matter. The view which is afforded of large opaque objects, however, by a Compound Microscope, furnished with even an imperfectly corrected Achromatic object-glass, giving a magnifying power of 20 or 25 diameters, is so greatly preferable to that which is given by any Simple Microscope, that no instrument that is intended for general research should be unfurnished with such a power. It is especially required in Microscopes that are to be used for Educational purposes; since it is most important that the young should be trained in a knowledge of the wonders and beauties of the familiar objects around them; and an objective of low power and wide aperture, adapted to the examination of a large surface at once, affords a means of displaying these, such as can be afforded in no other way, save by the use of the Erector and draw-tube (§ 44). A microscope furnished with these appendages, need not be supplied with an objective of longer focus than 1 inch or 8-10ths inch; but the Author would strongly recommend to such as do not possess them, that they should give to a dividing" 1 inch or 2 inch (in which the front lens is removable, and is replaced by a perforated cap that limits the aperture of the back lens, which is then employed by itself, having a focus of about 3 inches) a preference over such as do not thus supply the extremely low power which he recommends.'

92. The mode of bringing opaque objects under view, will differ according to their "mounting," and to the manner in which it is desired to illuminate them. If the object be mounted in a "slide" of glass or wood, upon a large opaque surface, the slide must be laid on the stage in the usual manner, and the object brought as nearly as possible into position by the eye alone (§ 84). If it be not so mounted, it may be simply laid upon the glass stage-plate, resting against its ledge; and the diaphragmplate must then be so turned, as to afford it a black background.

A single pair (flint and crown) of about 2 inch focus, was constructed at the Author's request, some years since, by Messrs. Smith and Beck, for the special purpose of exhibiting injected preparations, and other opaque objects; and its performance has been so satisfactory to him, that he was induced to urge upon the Microscopic Committee appointed by the Society of Arts, that the Educational Microscope for which they invited competition (§ 31), should be furnished with such a power. This recommendation having been adopted, the instrument selected has been specially fitted for the class of objects above alluded to.