Compound Microscope The limitation of a simple microscope is overcome in a compound microscope. In a compound microscope, to obtain a large magnification, first of all the magnified image of an object is obtained with the help of a lens, and then with the help of another convex lens more magnification is obtanied. Thus, the total magnification is obtained at two stages, which is very large.

Construction and Working A compound microscope consists of two converging (convex) lenses spaced apart and fitted co-axially in a tube. The lens near the object is called object lens (or objective), and the lens near the eye is called the eyepiece. The focal length and aperture of the objective are small while they are relatively larger for the eyepiece. The two lenses are adjusted in a tube in such a way that the image IR formed by the objective L1, works as an object for the eyepiece and its magnified image I' R' is seen at the least distance of distinct vision D. As illustrated in OP is an object placed in front of the objective L1, away from the focus. An image IR is formed by the objective which s a real, magnified and inverted image of the object OP. Now, the eyepiece is so adjusted that the image of IR is formed at the least distance of distinct vision (I'R') for an eye placed just behind the eyepiece. This image is virtual, magnified and erect with respect to IR, but inverted with respect to the object OP.

Since virtually no microscope object is self luminous, a means is provided for illuminating it. For transparent objects, a condenser is used whose purpose is to concentrate a cone of light on to the object. It must be capable of providing a cone of light at an angle which will fill the objective lens so as to make use of the full numerical aperture (NA) of the objective. In practice, the best results are usually obtained with a cone of light equal to about three quarters (3/4) of the objective NA.

A practical compound microscope used in industries. This compound microscope is provided with a fine-focusing adjustment, the range of which is of the order of a millimeter. Only one tenth of this range is normally used because the coarse adjustment is used to bring an object under a high-power objective very nearly into exact focus. These focusing adjustments move the body tube which has a standard length of 160 mm. The eyepieces are made to a standard diameter which slides into the upper end of the body tube. The objectives are provided with screw thread and fit into the aperture of a revolving nosepiece or objective changer. The object, normally mounted on a glass slip 75 mm X 25 mm, is held by clamping fingers on the surface of the mechanical stage. Just below the mechanical stage, controls are provided for moving the object in two directions, at right angles. Scales and Vernier, on the motions permit the recording of the position on the object of a particular detail and measuring. A condenser is mounted below the stage which carries an iris diaphragm for controlling the angle of illumination, and both are carried into a centring mount which allows the unit to be accurately centred to the objective. The whole unit can be focused by a rack and pinion on dovetail, in the same way as the coarse focusing adjustment.

Types There are two types of compound microscope:

( i ) Monocular and

( ii ) Binocular.

The basic principle and construction of monocular and binocular microscopes are the same; the only difference is that the binocular is fitted with a binocular head which consists of two eyepieces. The monocular and binocular bodies are interchanged on a special slide fitted with a cam-locking device which ensures positive and accurate alignment of the optical axis. Adjustment for interocular separation is provided and one eyepiece has the facility for individual focusing to adjust any difference in the eyes of the observer.

The optical system of the binocular body. A beam of light from the objective is divided into two equal parts (beams), one to each eyepiece, by the centre prism, and the remaining prisms direct the light along the two eyepiece tubes. Since the path travelled by the light is longer than in the monocular tube, two correcting lenses are provided to correct for this difference. The prisms and lenses do not cause any image deterioration, and their surfaces are coated to reduce reflection losses and maintain maximum image brilliancy. The two tubes of the binocular microscope are movable to facilitate changing their separation to fit interpupillar distance of the observer. There is usually an arrangement for individual focusing of one of the eyepieces. The non-adjustable eyepiece is focused in the usual way by moving the tube on the coarse and fine-adjustment mechanisms, and then the second eyepiece is focused to the individual eye.

Inclined bodies are sometimes furnished on binocular microscopes so that the objectives is vertical for the examination of liquids, etc. This is made possible throught he use of another inclining prism between the right-angle prisms (dividing set) and the objective. This inclining prism bends the beam of light through an angle of 45 degree but does not invert the image.

The advantage of a binocular microscope is that eye strain is completely eliminated, and the inclination of the eyepiece tubes gives comfort and convenience in working with the instrument in the vertical position.

Magnification Compound Microscope The magnification 'm' of the compound microscope.

Therefore, the total magnification is the product of the magnifications of the objective and the eyepiece.

The stereoscopic Microscope The stereoscopic microscope is essentially a binocular instrument of low power used for observations where the third dimension, depth, is of importance.

Construction It consists of two similar compound microscopes of low power with their axes inclined at an angle of about 15 degree. The spacing is such that the axes intersect on the focal point. Two prisms are provided to give erect images to avoid a pseudoscopic effect. These prisms can be rotated about the entering axes for adjusting the eyepiece separation. In research type of stereoscopic microscopes, inclined eyepiece tubes are provided and the nosepiece carries three pairs of parfocal objectives which are shroudedfor protection. The stand can rapidly be split into sections so as to adapt the instrument for special purposes.

Uses Stereoscopic microscopes are used for the following purposes:

( i ) They are generally used as surgical microscopes to aid the surgeon in certain delicate operations.

( ii ) They are also used in the adjustment and assembly of small mechanisms and in the checking of small components.