We have supposed the whole of the light to enter the eye through the lens AB (fig. 1), but we must now state that so large a pencil of light passing through a single lens would be so distorted by its spherical figure, and by the chromatic dispersion of the glass, as to produce a very indistinct and imperfect image. This is so far rectified by applying a stop to the lens, so as to allow only the central portion of the pencil to pass. But whiia such a limited pencil would represent correctly the form and color of the object, so small a pencil of light is unable to bear diffusion over the magnified picture, and is therefore incapable of displaying those or ganic markings on animals or plants which are often of so much importance in distin,g-uishing one class of objects from another. Dr. Wollaston was the first to overcome this difficulty, which II,. achieved by constructing a doub let (tig. 2) which consists of two plano-convex lenses, haviug their focal lengths in the pro portion of 1 to 3, and placed at a distance best ascertained by experiment. Their plane sides are placed towards the object, and the lens of shortest focal length next the object. By this arrangement the distortion caused by the first lens is corrected by the second, and a well-detined and illuminated image is seen. Dr. Wollaston's doublet was further improved by Mr. Holland, who substituted two lenses for the first in Dr. Wollaston's doublet, and retained the stop between them and the third. This combination, though ,generally called a triplet, is virtu ally a doublet, inasmuch as the two lenses only accomplish what the anterior lens did in Dr. -Wollaston's doublet, although with less precision.
In this combination (fig. 3) of lenses, the errors are still further reduced by the close approximation of the lenses to the object, which causes the refractions to take place near the axis, and thus we have a still larger pencil of light transmitted, and have also a more distinct and vivid image presented to the eye.
Simple ilieroscope.—By this term we mean au instrument by means of which we view the object through the lens directly. These instruments may be divided into two classes—those simply used iu the hand, and those provided with a stand or frame, so arranged as to be capable cf being adjusted by means of a screw to its exact focal dis tance, and of being moved over differeut parts of the object. The single lenses used may be either a double convex or a plano-convex. When a higher power is wanted, a doub let, such as we have already described, may- be employ-ed, or a Coddington lens which consists (fig. 4) of a sphere in which a groove is cut and filled up with opaque matter. This is perhaps the most convenient hand lens, as it matters little, from its spherical form, in what position It is held. In the simple micro scope. single or combined lenses may be employed, varying from to 2 inches. There are many different kinds of stauds for simple microscopes made, but as they are principally used for dissection, the most important point next to good glasses is to secure a firm large stage for supporting die objects under examination. When low powers alone are Used, the stage
movements may be dispensed with; but when the doublet or triplet is employed, some more delicate adjustment than that of the hand is necessary.
Compound Microscope. —1n the compound microscope the observer does not view the object directly, but au inverted image or picture of the object is formed by one lens or set of lenses, and that image is seen through another lens. The compound microscope consists of two lenses, an object and an eye lens; but each of these may be compounded of several lenses playing the part of one, as in the simple scope. The eye-lens is that placed next the eye, and the lens that next the object. The former is als,o called the ocular, and the latter the objective. The object-glass is generally made of two or three achromatic lenses, while the eye-piece generally consists of two plano-convex lenses, with their flat faces next the eye, and separated at half the sums of their focal len,g-tlis, with a diaphrag-rn or stop between them. Lenses of high power are so small as to admit only a very small beam of light, ttnd consequently what is gained in magnifying power is often worthless from deficient mination. Various devices have been employed to overcome this difficulty. The light may be concentrated by achromatic condensers placed beneath the stage, or the curvature of the lens may be such as to allow as large a number of divergent rays as possible to impinge upon it. Such a lens is said to have a large " angle of aperture, ' the angle of aperture being that made by- two lines converging from the margins of the lens to its focal point. Recently lenses, termed "immersion lenses," have been constructed, of such a curvature that when immersed in a drop of water placed over the object, light is admitted on all sides. With an inunersion lens, there is high fying power with sufficient illumination.
The following diagram (fig. 5) explains the manner in which the compound microscope acts. We have here represented the triple achromatic objective, consisting of three achromatic lenses combined in one tube, in connection with the eye-piece, which consists of the field-glass FF, and the eye-glass EE. Three rays of light are repre sented as proceeding from the center, and three from each end of the object. These rays would, if not interfered with, form an image at AA; but coming in contact with the field-glass FF, they are bent, and made to converge at BB, where the image is forme& at which place a stop or diaphragm is placed to Intercept all light, except what is required to form a distinct image. From BB, the rays pro ceed to the eye-glass exactly as they do in the simple microscope, and as we have explained in fig. 1. The image therefore formed at BB is viewed as an original object by an observer through the eye-piece EE. The lens FF is not essential to a compound microscope; but as it is quite evident that the rays proceeding to AA would fall without the eye-lens EE, if it was removed, and only a part of the object would thus be brou,ght under view, it is always made use of in the oompound microscope.