PHOTO-LITHOGRAPHY. This process, like the last, is in a some what imperfect state, but it will no doubt receive further development and become eventually of much practical utility. It consists in pro ducing from a negative a positive upon stone which will take printers' ink, and may be printed from in the same way as an ordinary litho graph. Two methods have been employed which are totally different in principle, since one depends upon the oxidizing, the other the de oxidizing power of light. The first method was described by Mr. Macpherson at a meeting of the Photographic Society of Scotland on Dec. 9th 1856, and was reported in " Photographic Notes," No. 18, as follows:— "Take a piece of bitumen of Judea, about 11 inch square ; pound it in a mortar till it is reduced to a very fine powder; then put it into a stopped bottle. Pour upon this, 6 ounces of pure sulphuric ether. Shake it for ten minutes, and then let it stand for five min utes till the powdered bitumen has fallen to the bottom, leaving the ether perfectly transparent, but of a rich brown colour. This liquid is useless, and must be poured off into a bottle, which it is desirable should be kept for such liquids as may afterwards be redistilled, and once more become pure and serviceable ether. To return to the original bottle, containing the sediment of bitumen :-6 ounces of pure ether must be again poured upon this bitumen. Shake the bottle well for a quarter of an hour, and then pour the contents of it through a ptipek filter into a clean stopped bottle. This liquid is now in a state for use, and the sediment remaining may be thrown away, as it can never again be used. I may mention that the liquid should be of a beautiful brown transparent colour. Take a lithographic stone, (the blue ones are better than the yellow—the difference between the blue and yellow tints being about the same as in'blue and yellow post paper) ;—see that the printer has prepared it with a very fine grain, as if for a chalk drawing. Place it upon a table, in a room with ordinary day-light,—but avoiding the sun's direct rays. Let it be levelled with a spirit-level, till it is perfectly horizontal. There should be no current of air in the room, and the operator should move as little as possible, either before or after the operation, that any movement of his body may not cause a current. His breath should also be held for a few seconds. He is then ready to begin, and for that purpose should pour out as much of the liquid into a wine glass, as will be required to cover the surface of the stone —a quantity which experience alone can teach him. He then pours it gently and slowly, but regularly, on to the centre of the stone. It immediately runs all over, and spreads itself ; and he must be again cautioned against withdrawing his arm quickly so as to produce a current of air. In less than a minute the whole of the ether will
have evaporated, leaving a dry looking light brown surface, which, on examination by a powerful lens, ought to appear granular, in other words ought to be composed of myriads of fine dots or stip ples.
" The negative, if of glass, must then be laid down upon the stone ; and great attention is required in laying it down, so that it may be at once put upon that part of the stone which is desired, for it must not afterwards be shifted, or the surface will be destroyed. It is then desirable to surround the negative with slips of pasted paper, so as to paste, and so attach, the negative closely to the stone, and in that way prevent air or dust getting between the stone and negative. It may then be exposed to the light. The exposure will be regulated by the experience of the operator. In my own ex perience it varies from half an hour to three days ; but I think the three days' exposure, without the sun, more likely to be successful than the half hour's exposure with the sun. The stone must then be removed into the operating room, the pasted slips of paper run round with a penknife, and the negative at once lifted cleanly off the stone. The operator can then more leisurely remove the slips of paper with a knife or other sharp instrument. A zinc bath is then placed horizontally on the table ; into the bath is poured as much ether as will give a depth of ir of an inch. The stone must then be let into the bath, face downwards, exactly as one silvers an albumenized plate—that is gently, but without hesitation or jerk. After it has been there for 8 or 10 seconds, it ought to be raised to an angle of 45° two or three times, up and down. Great dexterity is now required. The stone must be at one movement turned over face upwards, and the upper end of it made to rest upon the side of the bath, which ought not to be more than 3 inches high. At this part of the operation an assistant is required, who should be ready with a few ounces of pure ether, which he at once pours over the stone, with a continual movement back and forward, along the upper end of the stone. This ether runs rapidly down, washing away any loose particles of bitumen, which had not been perfectly removed by the washing in the bath. You then place the stone upon a table leaning against a wall, and you see before you the image upon the stone. Should the shadows instead of being dark appear light, and the lights appear dark, the operator will know that it is good for nothing, and that over-exposure was the cause of failure.