Simple Microscope

SIMPLE MICROSCOPE The distinctness with which the detail of an object can be seen depends on (a) the sharpness of the image produced in the eye and (b) the size of this image. As an object is brought nearer and nearer to the eye, it is seen more and more distinctly, pro vided the image is kept sharply focussed on the retina, because the image and image-detail get larger and larger. The upper limit of distinctness is reached when the object is in the nearest position at which its image can be sharply focussed. For the normal eye this distance is taken to be i o in. or 25 cm., and this is called the normal distance of clearest vision (fig. i.a). By using a simple microscope in front of the eye, it is possible to bring an object nearer than the distance of clearest vision for the unaided eye, and yet to see it clearly (fig. i.b).

If f is the focal length of the lens and D the observer's dis tance of clearest vision, the image seen is larger than the largest image that could be seen clearly by the unaided eye in the pro portion D/f. This is the magnifying power of the lens when used in this way. The assumed standard value for D, is the normal distance of clearest vision, i.e., 10 in. or 25 cm. We thus obtain appears all to be in focus at once. Actually only the points in one plane are truly focussed at any one time. If circular images on the retina are smaller than about in., however, they appear sharply defined, hence there is an appreciable depth of focus obtained with lenses of low power or very small aperture, owing to the small angle of the cone of rays entering the eye from any particular point in the object.

Common Forms of the Simple Microscope.---An

ordinary convex lens with spherical surfaces does not give a perfect image, If the distance between the object and the lens is made some what smaller than the focal length of the lens, the rays from the object may be made to enter the eye as though they, came from an enlarged object situated at the distance of clearest vision (fig. .c). When the lens is held quite close up to the eye and used in this way its magnifying power is given by Magnifying powers of lenses of comparatively long focus are given by the second formula, but the magnifying powers of short-focus lenses are calculated by the first formula. These

formulae really give nothing more than standard methods of rating a lens in terms of its magnifying properties. Only a person with a distance of distinct vision equal to io in. (i.e., only a normal-eyed person) will use a lens so as to obtain its rated magnifying power.

When an object is viewed through a fine hole in an opaque screen held close to the eye, the rays from any point in the object enter the eye in such narrow pencils that the image appears sharp, even when the object is well within the distance of clearest vision. A fine hole can thus be used, in a way, instead of a simple microscope.

Regulation of the Rays.

Simple microscopes of moderate or low powers are not usually fitted with diaphragms at all, as the pupil of the observer's eye determines what rays are effective in producing the image seen. The eye is usually placed close up to the simple microscope, so that by moving the eye about in its socket the images of different parts of the object can be brought on to the central portion of the retina, exactly as is done when the eye is examining an object without the aid of a lens.

In high-power lenses it is necessary to limit the rays which are allowed to pass through the lens, so as to reduce the effects of the aberrations of the lens. This is done by fitting one or more diaphragms, so that only those rays for which the lens is properly corrected can pass through. In order to examine different portions of an object by means of such a lens, it is necessary to move the eye from side to side by moving the whole head, exactly as though the object were being examined through a hole in a screen.

The brightness of an object as seen through a simple microscope remains practically constant as the eye is moved about, unless the rays from the particular portion being observed are partly cut off by the lens mounting, or by a diaphragm in the lens.