The simple hand magnifier are often employed by microscopist in the preliminary examinations of objects.

A very good form in lens was proposed by Dr. Wollaston, and called by him the Periscopic lens: which consisted of two hemispherical lenses cemented together by their plane faces, having a stop between them to limit the aperture. A similar proposal was made by Sir David Brester in 1820, who however executed the project in a better manner, by cutting a groove in a whole sphere, and filling the groove with opaque matter. His lens, which is better known as the Coddington lens gives a large field of view, which is equally good in all directions, as it is evident that the pencils a b and b a pass through under precisely the same circumstances. Its spherical form has the further advantage of rendering the position in which is held of comparatively little consequence. It is therefore very convenient as a hand magnifier; but its definition is, of course not as good as that of a well-made doublet or chromatic lens.

It is generally set is folding case, as represented in the figure, and so contrived that it is admirably adapted for the waistcoat pocket; which, together with the small holder, for securing small objects and holding them during examination, are all that is required for a field instrument during a day ramble. This useful little holder may be purchased in a case. The Stanhope lens is similarly constructed, although not so good and convenient as the former, and is but seldom to be purchased properly made.

When the magnifying power of a lens is considerable, or when its focal length is short, and its proper distance from the object equally short, it then becomes necessary to be placed at a proper distance with great precision; it cannot therefore be held with sufficient accuracy and steadiness by the unassisted hand, but must be mounted in a frame, having a rack or screw to move it towards or from another frame or stage which hold the object. It is called a microscope; and it is furnished, according to circumstances, with lenses and mirrors to collect and reflect the light upon the object, with other conveniences.

The best of kind was that contrived by Mr, Ross, represented; and consists of a circular foot, from which rises a short tubular stem, into which slides another short tube, carrying at its top a joint; to this joint is fixed a square tube, through which a rod slides; this rod has at once end another but smaller joint, to which is attached a collar, for receiving the lens. By means of the join at, the square rod can be moved up or down, so as to bring the lens close to the object; and by the rod sliding through the square tube, the distance between the stand and the lens may be either increased or diminished: the joint, at the end of the rod, is for the purpose of allowing the lens to be brought either horizontally or at an angle to the subject to be investigated.

By means of the sliding arm the distance between the table and the jointed arm can be increased or diminished. The microscope is provided with lenses on one-inch and half-inch focal length, and is thereby most useful for the examination and dissection of objects. It is readily unscrewed and taken to pieces, and may be packed in a small case for the pocket.