Another form of simple lens was devised by Dr. Wollaston, the “Periscopic.” This combination consists of two hemispherical lenses cemented together by their plane faces, with a stop between them to limit the aperture. A similar proposal, made by Sir David Brewster in 1820, is known as the Coddington lens,[19] shown at Fig. 50: this has a somewhat larger field, and is equally balanced in all directions, as is made evident, the pencils _a b_ and _b a_ passing through under precisely the same circumstances. Its spherical form has the further advantage of rendering the position in which it is held of comparatively little consequence. It is still used as a hand magnifier, although its definition is certainly not so good as that of a well-made doublet. It is usually set in a folding case, as represented in the figure, and so contrived as to be admirably adapted for the waistcoat-pocket. It is usually sold with the small _holder_, Fig. 50_a_, for holding and securing small objects during examination. Browning’s Platyscopic Pocket Lens is a useful form of pocket lens for the botanist and mineralogist. Its focus is nearly three times longer than that of the Coddington, and allows of opaque objects being more easily examined; it has also a magnifying power of 15, 20, and 30 diameters. [Illustration: Fig. 50.--The Coddington Lens.] [Illustration: Fig. 50_a._] One of the best combinations of the hand or pocket form of lens is that known as _Steinheil’s aplanatic lens_ (Fig. 51); it consists of a bi-convex lens cemented between two concavo-convex lenses, giving a relatively long focal distance and a large flat field. The higher powers of this lens are much used for dissecting purposes. This handy magnifier appears to have suggested a later combination, the apochromatic of Zeiss. No hand lens can compare with Steinheil’s “_loups_.” [Illustration: Fig. 51.--Steinheil’s Aplanatic Lens.] [Illustration: Fig. 52.--Simple Microscope.] When the magnifying power of a lens is considerable, or when its focal length is short, or it is wished to use it with greater precision and steadiness, it should be mounted on a short stand with a tubular stem, with rack-work focussing movement and mirror illumination. Fig. 52 represents a simple dissecting microscope, with a glass circular stage, 4-1/2 inches in diameter, supported on three legs--a handy and useful form of instrument for many purposes. The Compound Microscope.