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 which meet--having a focal length of 18 inches. For every distant view, the aperture in front is contracted by a diaphram to 1/8 of an inch. By this means the light is reflected with considerable intensity and the clearness and correctness of the pictures are truly surprising. THE AMERICA instruments are constructed on the same principle and many of them are equally perfect. Mr. Edward Anthony of 205 Broadway, New York city, has constructed, and sold cameras fully equal to the German and for which Voitlander instruments have been refused in exchange by the purchaser. The ordinary camera box (see fig. 5, a) varies in size to suit the tube, and is termed medium, half, or whole. Within the box is a slide to assist in regulating the focus, and in enlarging or diminishing the picture. In one end of this slide is a springed groove into which the ground-glass spectrum (g fig. 5) is slid, for the purpose of more conveniently arranging the focus. After the plate is prepared it is placed in the holder--partly seen at e, fig. 5, and covered with the dark slide f, fig. 5; the spectrum is then withdrawn and the holder takes its place, and the lids d, d, are closed after removing the dark slide f. The plate is now ready to receive the image, and the cap c may be removed to admit the light into the box. A camera constructed by Voitlander is thus described by Mr. Fisher. "It is made entirely of brass, so that variations of climate has no effect upon it. It is very portable and when packed in its box, with all the necessary apparatus and materials for practising the Daguerreotype art, occupies but very little space. It is not, however, well adapted for the Calotype process." [Illustration: Fig. 7 (hipho_7.gif)]