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. Read the rest of this entry »

To assist us in gaining a clearer notion to the mode in which a single lens serve to magnify minute objects, it is necessary to take a passing glance at the ordinary phenomena of vision. The human eye is so constituted, that it can only have distinct vision when the rays falling upon it care parallel or slightly divergent; because the retina, on which the image impinges, requires the intervention of the crystalline lens to bring the rays to an accurate focus upon its surface. Read the rest of this entry »