The original calotype picture, like the photographic one described in the last chapter, is negative, that is to say, it has its lights and shades reversed, giving the whole an appearance not conformable to nature. But it is easy from this picture to obtain another which shall be conformable to nature; viz., in which the lights shall be represented by lights, and the shades by shades. It is only necessary to take a sheet of photographic paper (the bromide paper is the best), and place it in contact with a calotype picture previously rendered transparent by wax or oil as before directed. Fix it in the frame, Fig. 29, expose it in the sunshine for a short time, and an image or copy will be formed on the photogenic paper. The calotype paper itself may be used to take the second, or positive, picture, but this Mr. Talbot does not recommend, for although it takes a much longer time to take a copy on the photogenic paper, yet the tints of such copy are generally more harmonious and agreeable. After a calotype picture has furnished a number of copies it sometimes grows faint, and the subsequent copies are inferior. This may be prevented by means of a process which revives the strength of the calotype pictures. In order to do this, it is only necessary to wash them by candlelight with gallo-nitrate of silver, and then warm them. This causes all the shades of the picture to darken considerably, while the white parts are unaffected. After this the picture is of course to be fixed a second time. It will then yield a second series of copies, and, in this way, a great number may frequently be made. The calotype pictures when prepared as we have stated, possess a yellowish tint, which impedes the process of taking copies from them. In order to remedy this defect, Mr. Talbot has devised the following method. The calotype picture is plunged into a solution consisting of hyposulphite of soda dissolved in about ten times its weight of water, and heated nearly to the boiling point. The picture should remain in about ten minutes; it must then be removed, washed and dried. By this process the picture is rendered more transparent, and its lights become whiter. It is also rendered exceedingly permanent. After this process the picture may be waxed, and thus its transparency increased. This process is applicable to all photographic papers prepared with solutions of silver.