Compound Microscope

COMPOUND MICROSCOPE A compound microscope con sists of a positive or converging lens, called theobject-glass, which forms a real, inverted, magni fied image of the object. This image is then viewed by means of a second lens, called the eyepiece, which is usually another positive lens or lens system (fig. I 1). For certain purposes a negative lens is sometimes used as the eyepiece; this cause an "erect" image to be seen (fig. 12).

If we consider the object-glass as a thin lens, with the object at a distance d on one side of it and the image at a larger distance D on the other side, then the image will be larger than the object in the proportion D/d. This propor tion D/d is the magnification, Mo,produced by the object-glass. The eyepiece may be consid ered as a simple microscope, of magnifying power ME , which is used to examine the magnified image formed by the object glass. The total magnification obtained is thus equal to Mo X ME.

The distance d is approximately equal to the focal length of the object-glass, while D may be taken as equal to the "tube length" (L) of the microscope. This gives Mo equal to L/fi. The value of ME is taken as where is the focal length of the eyepiece in inches (see Simple Microscope). Hence, if all the quantities are expressed in inches, the magnification obtained is given approximately as With an object-glass and an eyepiece, each of one inch focus, therefore, the magnification obtained, if the tube length is 1 o in., would be about i oo, or, with a tube length of 6 in., about 6o.

The quality of the image seen in the compound microscope depends primarily on the quality of the real image formed by the object-glass. Any imperfections in this image are magnified when the image is viewed through the eyepiece. The image formed by the object-glass should, therefore, be as sharp as possible, and to secure this the aberrations of the object-glass should be reduced to the smallest practicable values. The object-glass should also have a high resolving power.

Resolving Power.

Imagine the lens L (fig. 13.a.) to be a perfectly corrected lens with its aperture limited by a diaphragm, the edges of which are at DD. A luminous point is at 0, on the axis of the lens, and an image of this point is formed at I by the lens. This image will not be a point but, in consequence of the diffraction of the light at the diaphragm, will take the form of a bright disk surrounded by concentric dark and bright rings (see LIGHT). The brightness of the central disk and of the surround ing system of rings is indicated in fig. 13.b, in which the form of the curved line represents the distribution of brightness in the image plane around I. The diameter of the first dark ring, or the full diameter of the central disk, is represented by the distance dd. The brightness of the disk falls off rapidly towards the edge, and a visual estimate of the diameter would usually put it at about of the full diameter dd. The diameter dd depends on the wave-length of the light emitted by the source at 0, and on the angles which the extreme rays make with the axis of the lens.

If we have two independent similar point sources at 0 and 0' (fig. 13.c) equidistant from the lens, each of these will produce a disk image with its surrounding system of rings ; the centres of the two systems will be at 1 and 1' respectively. If the disks are completely separated they will be seen clearly, but if they over lap by more than a certain amount they will merge into a single bright area over the centre of which the brightness is almost uni form: the two disks will not then be distinguishable as separate images. In the latter case the resolving power of the lens is insufficient to enable 0 and 0' to be resolved as separate point sources. The resolving power of a microscope object-glass is usually stated in terms of the minimum distance which must exist between two details in the object if their images are to be distinguishable as separate images.