For this purpose we must quit the warm precincts of the cheerful day, and go into the narrow den where the deeds of darkness are done. Its dimensions are of the smallest, and its aspect of the rudest. A feeble yellow flame from a gas-light is all that illuminates it. All round us are troughs and bottles and water-pipes, and ill-conditioned utensils of various kinds. Everything is blackened with nitrate of silver; every form of spot, of streak, of splash, of spatter, of stain, is to be seen upon the floor, the walls, the shelves, the vessels. Leave all linen behind you, ye who enter here, or at least protect it at every exposed point. Cover your hands in gauntlets of India-rubber, if you would not utter Lady Macbeth's soliloque over them when they come to the light of day. Defend the nether garments with overalls, such as plain artisans are wont to wear. Button the ancient coat over the candid shirt-front, and hold up the retracted wristbands by elastic bands around the shirt-sleeve above the elbow. Conscience and nitrate of silver are telltales that never forget any tampering with them, and the broader the light the darker their record. Now to our work. Here is a square of crown glass three-fourths as large as a page of the "Atlantic Monthly," if you happen to know that periodical. Let us brush it carefully, that its surface may be free from dust. Now we take hold of it by the upper left-hand corner and pour some of this thin syrup-like fluid upon it, inclining the plate gently from side to side, so that it may spread evenly over the surface, and let the superfluous fluid drain back from the right hand upper corner into the bottle. We keep the plate rocking from side to side, so as to prevent the fluid running in lines, as it has a tendency to do. The neglect of this precaution is evident in some otherwise excellent photographs; we notice it, for instance, in Frith's Abou Simbel, No. 1, the magnificent rock-temple facade. In less than a minute the syrupy fluid has dried, and appears like a film of transparent varnish on the glass plate. We now place it on a flat double hook of gutta percha and lower it gently into the nitrate-of-silver bath. As it must remain there three or four minutes, we will pass away the time in explaining what has been already done.