But the principal question in regard to illumination is the mag heads arc at c. By means of the large milled head D, a triangular bar, having a mck,is elevated out of the stem A, carrying the lens-holder E, which bat; a horizontal movement in one direction, by means of a rack worked by the milled head F, and in the other direction by turning on a circular pin. A concave mirror a reflects the light upwards through the hole in the stage, and a lens may be attached to the stage for the purpose of throwing light on an opaque object, in the same way that the forceps u for holding such objects is attached. This microscope is peculiarly adapted, by its broad stage and its general steadiness, for dissecting; and it is rendered more convenient for this purpose by placing it between two inclined planes of mahogany, which support the arms and elevate the wrists to the level of the stage. This apparatus is called the dissecting rest. When dissecting is not a Mtn& of the illuminating pencil, particularly in reference to trans. parent objects. Generally speaking the illuminating pencil should be as large as can be received by the lens, and no larger. Any light beyond this produces indistinctness and glare. The superfluous light from the mirror may be cut off by a screen haviug various-sized apertures placed below the stage; but the best mode of illumination is that proposed by Dr. Wollaston, and called the Wollaston condenser. A tube is placed below the stage of the instrument containing a lens A 13 (fig. 11), which can be elevated or depressed within certain limits at pleasure; and at the lower end is a stop with a limited aperture c D. A plane mirror E F receives the rays of light L L from. the sky or a white cloud, which last is the best source of light, and reflects them upwards through the aperture in c D, so that they are refracted, and form an image of the aperture at o, which is supposed to be nearly the place of the object. The object is sometimes best seen when the image of the aperture is also best seen ; and sometimes it is best to elevate the summit a of the cone A D o above the object, and at others to depress it below : all which is done at pleasure by the power of moving the lens An. If artificial light (as a lamp or candle) be employed, the flame must be placed in the principal focus of a large detached lens on a stand, so that the rays L L may fall in parallel lines on the mirror, or as they would fall from the cloud. This will be found an advantage, not only when the Wollaston condenser cmploycd,but also when the mirror and diaparagm are used. A mode of imitating artificially the light of a white cloud opposite the sun has been proposed by Mr. Varlcy : he covers the surface of the mirror under the stage with carbonate of soda or any similar material, and then concentrates the sun's light upon its surface by a large con densing lens. The Intense white light diffused from the surface of the soda forms an excellent substitute for the white cloud, which, when op poelte the sun and of considerable size, is the boat daylight, as the pure sky opposite to the sun is the worst.
Tde Ceweresold Jfirreseope may, as before Rated, consist of only two lenses, while a simple microscope has been shown to contain sometime,' three. In the triplet for the simple microscope, however, it was explained that the effect of the two first lenses was to do what might have been accomplished, though not so well, by one ; and the third merely effected certain modifications in the light before it entered the eye. But in the compound microscope the two lenses have totally different functions ; the first receives the rays from the object, and, bringing them to now foci, forms an image, which the second lens treats as an original object, and magnifies it just as the single micro scope magnified the object itself.
The anuexed figure (12) shows the course of the rays through a com pound microscope of two lenses. The rayi proceeding from the object A a are so acted upon by the lens c o, near it, and thence called the objective, that they are converged to foci in Ale, where they form an enlarged image of the object, as would be evident if a piece of oiled paper or ground glass were placed there to receive them. They are not so intercepted, and therefore the image is not rendered visible at that place ; but their further progress is similar to what it would have been had they really proceeded from an object at A're. They are at length re ceived by the eye-lens L, fa, which acts upon them as the simple microscope has been described to act on the light proceeding from its objects. They are bent so that they may enter the eye at E in parallel lines, or as nearly so as is requisite for distinct vision. When we say that the rays enter the eye in nearly parallel lines, we mean only those which proceed from one point of the original object. Thus the two parallel rays at E have proceeded from and are part of the cone of rays c A D, emanating from the point A of the arrow; but they do not form two pictures in the eye, because any number of parallel rays which the pupil can receive will be converged to a point by the eye, and will convey the impression of one point to the mind. In like manner the rays Le are part of the cone of rays emanating from a, and the angle L E M is that under which the eye will see the magnified image of the arrow, which is evidently many times greater than the arrow could be made to occupy in the naked eye at any distance within the limits of distinct vision. The magnifying power depends on two circum stances : first, on the ratio between the anterior distance A c or B 1) and the posterior foal length c u or n A'; and secondly, on the power of the eye-lena L x. The first ratio is the same as that between the object A B and the Image A'n'; this and the focal length or power of the eye-lens are both easily obtained, and their product is the power of the com pound instniment.
Since the power depends on the ratio between the anterior and posterior foci of the objective, it is evident that by increasing that ratio any power may be obtained, the same eye glass being used ; or, having determined the first, any further power may be obtained by increasing that of the eye-glass ; and thus, by a pre.arrangement of the relative proportions in which the magnifying power shall be divided between the objective and the eye-glass, almost any given distance (within certain limits) between the first and Its object may be secured. This is one valuable peculiarity of the compound instrument ; and another is the large field, or large angle of view, which maybe obtained, every part of which will be nearly equally good ; whereas with the best simple microscopes the field is small, and is good only in the centre. The field of the compound instrument is further increased by using two glasses at the eye-end; the first being called, from its purpose, the held-glass, and the two constituting what he called the eyepiece. This will be more particularly explained in the figure of the achromatic compound microscope presently given.