on the distance of the object, and also on the superficies of the plate or paper to be covered. For portraits one of 1 1/2 inches diameter, and from 4 1/2 to 5 1/2 inches focus may be used; but for distant views, one from 2 inches to 3 inches diameter, and from 8 to 12 inches focal length will answer much better. For single lenses, the aperture in front should be placed at a distance from it, corresponding to the diameter, and of a size not more than one third of the same. A variety of movable diaphrams or caps, to cover the aperture in front, are very useful, as the intensity of the light may be modified by them and more or less distinctness and clearness of delineation obtained. These caps alway come with Voitlander instruments and should be secured by the purchaser. Though the single acromatic lens answers very well for copying engravings; taking views from nature or art, for portraits the double should always be used. The extensive manufacture of the most approved cameras, both in Europe and in this country, obviates all necessity for any one attempting to construct one for their own use. Lenses are now made so perfect by some artisans that, what is called the "quick working camera" will take a picture in one second, while the ordinary cameras require from eight to sixty. The camera in most general use is that manufactured by Voitlander and Son of Germany. Their small size consists of two seperate acromatic lenses; the first, or external one, has a free aperture of 1 1/2 inches; the second, or internal, 1 5/8 inches; and both have the same focus, viz: 5 3/4 inches. The larger size differs from the smaller. The inner lens is an achromatic 3 1/4 inches diameter, its focal length being 30 inches. The outer lens is a meniscus--that is bounded by a concave and convex spherical surface