“positive” picture, which is done by placing the negative on a piece of prepared paper, and exposing it to the light. The negative having been made partly transparent by wax (as will be further explained), and the parts of the picture which are to be dark being left transparent, the light passes through them and blackens those parts of the prepared paper behind, while those which are to be light, being dark in the negative, exclude the light and thus preserve the paper at the back from being darkened; the paper is thus affected in the contrary way to the negative, and is therefore a true picture of the lights and shadows of the object copied. This is the principle of all photographic pictures, but with numberless variations in its practice; it would therefore be impossible here to describe the particulars of each process, beyond the most simple. Illustration: FIG. 4. (‡ PHOTOGRAPHIC PLATE.) Perhaps of all the various arts discovered by man, this is the most beautiful, producing a perfect likeness of any being or object in a few moments, and with a truth not to be equalled by years of study and practice in other ways. One of the earliest discovered and simplest modes of forming a sun picture consists in using the object itself (where this is possible) to form the negative. This can only be done when the object is flat, as a manuscript, a picture, a leaf, or a piece of lace-work; a piece of paper washed over with a solution of common salt, dried, and again washed over with a solution of nitrate of silver, will answer for this purpose; it should be prepared by candle-light, as daylight blackens it, and it should be kept from the light till the moment for use. If this paper, having the object to be copied placed on it and kept flat by a piece of glass pressed firmly on by a contrivance shown in fig. 4, be exposed to the sunshine, it will be blackened in all places but those kept from the light by the darker parts of the object, and the lights and shadows will vary accordingly as the various parts of the object itself are more or less transparent. When the effect has been produced to a sufficient degree, the paper should be removed and at once washed in a solution of hyposulphite of soda, which removes all the superfluous silver (which otherwise would be darkened by the light); when removed from the hyposulphite of soda it should be well soaked in clean water. This when finished is a true negative picture of the object, and can be used to produce the positive. The negative should first be made partly transparent by brushing it over with some melted white wax, then placing it between folds of blotting paper and passing a hot iron over it, the blotting paper will absorb all the superfluous wax, and the negative will be fit for use. A piece of paper prepared like the first with salt and nitrate of silver, should be placed against the front of the negative, kept smooth by a piece of glass, and exposed to the sunshine for a quarter of an