Direct Positives are to be varnished, first with a layer of transparent varnish, and then with black japan. Suggett's patent jet is sometimes employed, but it has a disagreeable smell, and is apt to crack on drying. The best black japan used by coachmakers is more elastic and less liable to crack. Asphalt (4 oz.) dissolved in mineral Naphtha (10 oz.), with the addition of 30 grains of Caoutchouc dissolved in half an ounce of the same menstruum, is also said to stand well. A third formula contains black sealing-wax dissolved in Alcohol. In either case it will be best to apply first a layer of clear varnish to the film, and afterwards the black varnish, which should combine with the other without dissolving it. Positives whitened with Bichloride of Mercury are injured by varnishing; they must therefore be backed up with black velvet, or Japan laid upon the opposite side of the glass. Many prefer taking the picture upon coloured glass, using only a layer of clear varnish; but in this case the Collodion side being left uppermost, the image is necessarily reversed. SECTION II. _Directions for the use of Photographic Lenses._ Those who are comparatively unacquainted with the science of optics require simple rules to guide them in the choice of a photographic lens, and in the proper mode of using it. Two kinds of Achromatic lenses are sold, the Portrait lens and the View lens; the former of which is constructed to admit a large volume of light, for the purpose of copying living objects, etc. A convenient-sized Camera for small portraits is "the half-plate" with a lens of about 2-1/4 inches diameter, and giving a tolerably flat field on a surface of 5 inches by 4. Much however in this respect will depend upon the quality of the glass and also upon its focal length; a short focus lens taking a picture more quickly, but giving a smaller image, and a field which is misty towards the edge. There is also a great tendency to _distortion_ of the image in portrait lenses of large aperture and short focus, such as those employed for operating in a dull light. The "whole plate" portrait lens may be expected to cover 6-1/2 by 4-3/4 inches, and has a diameter of about 3-1/4 inches. It will take larger pictures than the last, but not necessarily in a shorter time; since, although the aperture for admitting the light is larger, the focal length is proportionately greater and the light less condensed. The "quarter-plate" portrait lens of 1-1/4 inch diameter is useful for stereoscopic subjects and small portraits; which are usually more sharply defined when taken with a small lens. The distance at which the Camera is to be placed from the sitter in taking a portrait, will depend upon the focal length of the lens. The effect of bringing the Camera nearer is to add to the size of the image, but at the same time to increase the chance of distortion; hence with every lens of full aperture, there is a practical limit to the size of picture which can be taken.