When the object to be viewed is transparent, the light is made to fall in a condensed state upon it, after having.been transmitted through a convex lens, or two such ; and, from the object, the rays proceed as before to the screen through the system of lenses which constitute the compound objective of the microscope, and through those at the opposite extremity of the box.
Instead of having a plate of ground-glass to receive the image, the pencils of light from the object, after passing through the lenses, may be allowed to fall on a board painted white or covered with white paper ; the distance of the board from the instrument (about 6 or 8 feet) being such that the rays in each pencil may converge to a point upon .
on it • thus there will be obtained a greatly magnified image of the object, which may be observed by many spectators at oue time. Such a screen should have the form of a segment of a hollow sphere, the light being received on its concave surface; for by that construction the image will be nearly equally distinct at the centre and about the margin.
The Solar Microscope, which was invented by Dr. Licherkhun, in 1738, produces a greatly magnified image of a small object in a similar manner by means of the sun's light. It consists of a conical tube fixed by its base to a frame of wood; the latter being screwed to a closed window-shutter at an aperture purposely made in the latter : the tube projects into the room, which, when the observations are to be made, is rendered quite dark, and is sometimes lined with black cloth. The magnifying power is produced by a system of lenses contained in the tube, as in other microscopes. On the exterior of the window is a frame carrying a rectangular piece of looking-glass ; this frame is attached by a joint at one of its sides to a ring of wood or brass which is made he object o is reflected by the inclined face of the mirror ar, and the ays are again reflected and converged by the ellipsoidal reflector n a, to surround the aperture in the shutter. The mirror, by turning on the hinge, is capable of being fixed at any angle with the wall of the building ; while the ring to which it is attached can, by means of a reek and pinion, be made to turn on the horizontal axis of the instru ment, so as to permit the rays of the sun, whatever be the position of the latter, to be reflected into the tube. By means of lenses disposed for the purpose, these rays are made to converge on the object; and from thence after refraction through the system of object-glasses, they proceed to a screen on which they depict the magnified image.
When the object is transparent, the rays of solar light are allowed to pass from the mirror directly through the lenses to the screen ; but when it is opaque, a convex lens placed at the aperture in the window shutter causes the solar rays to condense on a small mirror placed in a box at that end of the Instrument which is within the room, and from this mirror the rays are reflected to the object. The condensed light thus thrown, on the object diverges from thence and passes through the system of lenses, by refraction in which the magnifying power is produced; these lenses being placed in a tube which, as well as the: object, is within the box containing the mirror Lust mentioned, but a little above the latter, so that the rays of light proceeding from the object to the magnifying lenses may not be intercepted.
The mirror by which the sun's light is reflected into the instrument is sometimes connected with a clockwork apparatus, by which its posi tion is continually varied correspondently to the apparent change of the sun's place ; and thus the reflected light is made constantly to pass through the tube. [11 E L1OSTAT.)
Sir David Brewster has given, in his Treatise on New Philosophical Instruments,' p. 405, 4tc , a method of preparing objects of natural history for observation by the microscope, that their parts may pre serve their proper shape and colour, and thus be seen to the greatest advantage.
In the oxy-hydrogen microscope which is now more generally used, the chief effect is to throw an intense light upon the object, which is sometimes done by mirrors, and sometimes by lenses.
In Pig. 26, t. represents the cylinder of burning lime, n tt the reflector, irinrilanr. There aro four tenses peculiar to the passive signification, namely, the two futures in Ole*az and 'Kamm, and the two aoriste in Bev and Tie (eme-O,eseiai, rer...icroseu, ireep-env, drier-me. The future in cepa. (rk-oauca, alaseeseu), which is called in most grammars the future middle, has a passive signification, as well as a middle. (Monk on F.urip. !Eppel' I. 1458 ; ' Quarterly Journal of Education,' vol. iv. p, 158.) The following table will make the matter dearer :— Since then so many tenses in the Greek verb have the same form both for the middle and passive signification, it becomes en interesting object of inquiry, whether we should assign the priority to the passive or middle notion. This question has usually been answered by grammarians in favour of the passive ; but the comparison of other languages etymologically connected with the Greek, would lead us to a contrary supposition. Very few of the Indo-Germanic languages have a form peculiar to the passive signification ; even in Sanskrit the passive verb is not considered by Hindu grammarians as a distinct voice, but is classed among tho derivative verbs. There are however two voices in the Sanskrit verb, answering to the Greek active and middle, which are called respectively paresmaipadast and aintaelpa. duns ; the former answering to the active in Greek, and the latter having generally a reflective or medial but never a passive signification. The tenses of the passive verb are formed by prefixing the syllable ya to the person endings of the atmantpulum conjugation. These person endings are evidently the same as those of the middle voice in Greek, as the following table of the present tense will show :— which concentrates the light upon the object o o; the rays from which, passing through the two piano-convex lenses, aro brought to foci upon a screen placed at a great distance, and upon which is formed the mag nified image.
Fig. 27 shows a combination of lenses to condense the light upon the object. In either case the optical arrangcmente by which the image is formed admit of the same perfection as those which have been described for the compound microscopes. A few achromatic glasses for oxy-hydrogen microscopes have been made, and they will ultimately become valuable instruments for illustrating lectures on natural history and physiology. One inade by Mr. floes was exhibited a few years ago at the Society of Arts to illustrate a lecture on the physiology of woods. It should Le observed however that both the oxy-hydro gen and radar microscopes require either a spherical screen, or that the ob)ecte should be mounted between spherical glasses, in order to bring the whole into focus at one time. This latter plan was adopted on the occasion just mentioned with perfect success.