Simple Microscope

With high power lenses the light is often caused to pass through an area smaller than the pupil of the eye. There is then a loss of brightness in the image, and it may be necessary to illuminate the object strongly.

Depth of Focus.

In using a simple microscope to examine a three-dimensional object, different levels in the object can be focussed by making use of the accommodation of the eye, though this range is only very small with lenses of high power. With lenses of low power, or with lenses which have very small aper tures, there is also a certain range of depth in the object which owing to the aberrations of the lens. The aberrations with which we are chiefly concerned are (I) chromatic aberration, (2) spher ical aberration, (3) astigmatism and (4) flatness of field (see OPTIcs). To obtain a perfect image it is necessary, therefore, to correct the lens (I) so that it will bring light of all colours to a common focus, (2) so that each zone of the lens will have the same focus as the other zones, (3) so that each oblique pencil of light will come to a point focus after passing through the lens, and (4) so that points lying in one plane in the object will all appear sharply focussed at one time. These corrections can only be carried out by combining together lenses of different powers and made of different glasses, though spherical aberration can be appreciably reduced without using more than one lens.

Among the early lenses which were constructed so as to have reduced spherical aberration is Wollaston's. Wollaston made his lens as two almost hemispherical lenses with a diaphragm mounted centrally between their plane faces. Sir David Brewster improved on this by embedding a diaphragm in a transparent cement used between two hemispheres of glass. He obtained a precisely similar result by grinding a groove round a sphere of glass. Coddington did much towards making this type of lens more widely known and it is generally called the Coddington lens, though Coddington did not invent this nor did he claim to have done so. Brewster also in vented a lens in the form of a glass cylinder with its ends ground and polished as spherical surfaces of different curvature. When the more convex end is placed towards the eye, any object held in contact with the less curved end can be seen clearly. This form is usually known as the Stanhope lens.

Among early compound lenses are Wollaston's doublet which consists of two plano-convex lenses the focal lengths of which are in the ratio 3 : I, and Fraunhofer's lens consisting of two plano convex lenses of equal power with their convex sides mounted inwards and separated somewhat from each other. Wollaston's

doublet was improved later (I) by introducing a diaphragm be tween the two lenses, (2) by increasing the separation between the lenses, and (3) by substituting two lenses for the smaller lens, thus converting the system into a triplet. The triplet form was used when very high magnifying powers were required.

To correct for chromatic and spherical aberration simultane ously, positive and negative lenses of different powers and made of different types of glass are used in combination as doublet or triplet systems. Combinations of such systems can also be used. Steinheil's aplanatic is a typical triplet system of two negative lenses combined with a single positive. By using suitable combina tions of lenses, magnifying glasses having powers X 40 can be made practically free from aberrations, and satisfactory lenses giving X 7o and even X 10o have been produced. A type of lens which, though used as a simple microscope, is designed on the same lines as one form of compound microscope, was suggested by Chevalier and made by Briicke. It consists of a positive achro matic doublet, with a negative lens mounted at some distance behind it.

Stands.

For certain pur poses, such as dissection work, it is convenient to mount the simple microscope on an arm which is adjustable relative to a stage on which the object is laid. If a high-power simple microscope is being used, provision must be made for accurate focussing and for adequate illumination of the object. Usually, however, only low-power work is done nowadays with a simple microscope, and for this reason the stands are not generally provided with more than a rack and pinion adjust ment, with a mirror beneath the stage for illuminating transparent objects, and with a condensing lens for illuminating opaque objects. Detachable arm rests are fitted to the more complete models and the lens holder is adapted to take lenses of differ ent powers, which are inter changeable one with another.

The stage is either made of plate glass, or as a frame covered with a white opal-glass plate.

For dissecting and similar work it is of advantage if the details of the object can be seen as they are actually disposed in space. To meet this requirement stereo scopic devices are used.