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JAXUARY 9, 1885.j THE PHOTOGRAPHIC NEWS. 21 price of execution is from twopence the square centimetre, and the process is very rapid, and can be done in one day, and the printing is carried on the same way as copper-plate engraving. Comparing its price with that of wood engraving, the latter costs here, for anything like good work, from Is. to Is. 8d. the square centimetre, according to the subject; inferior work comes to about 3}d. or 4d. This aqua-tint process gives less depth of tone than the phototypes, but very great transparency in the shadows, without taking away from the vigour of the original. I may just mention that the price of the photogravures is 1 fr. per square centimetre, and a plate can be had in from a fortnight to three weeks’ time. I should never stop writing on this subject were it not that a question of colour has come up to feed my ambition, so great is it. Colour Sensitometer.—It becomes more than ever indis pensable, in comparing various sensitive films with a view of determining their relative actinic properties, to see how they are affected by light of various colours. Up to now one has been content with indicating the sensitometric degree of the rapidity of variously prepared plates; but it is certain that if isochromatic plates had been tried with other emulsions, the lengthened scale of colours reproduced by the former would have been obliged to have been taken into account. I constructed a colour-scale having twenty- one divisions, representing the seven distinct colours of the spectrum in three degrees of transparency. It is made of coloured glass, and is of the same size as Warnerke's sensitometer, so that two successive experiments can be made, one to give the degree of rapidity, and the other the scale of coloured rays more or less definitely repro duced. On making the trial with Tailfer and Clayton’s isochromatic and Britannia plates, equal rapidity and scale of coloured rays of equal length were obtained: while Monckhoven s, and plates by other makers, resist the luminous action under the yellow, orange, and red, show but little of the green, and display strong impressions under the violet, indigo blue, and greenish blue. The original I have made may be imitated ; it is not absolutely necessary to have a standard of sensitiveness to colour. Sensitometer Experiments. —I operated in an identical manner with Warnerke's phosphorescent plate and a stearine candle. My last trials were with a stearine candle of the brand known as I'eloile. It was placed exactly fifty centimetres away from the sensitive plate, and the base of the wick is kept in the same position. The exposure lasted exactly half-a-minute, the candle being allowed to burn several minutes, so as to arrive at the degree of normal combustion. It was repeated several times with plates of the same kind, and the results all agreed. I am therefore assured that, for all practical purposes, Siemens’ unit lamp, aud other complications, are unnecessary. A good candle of known diameter is all that is wanted. I have tried all kinds and the best give equal results. I learn that Mr. Warnerke is about to establish a new form of sensi tometer. which he will guarantee invariable. I congratu late him upon it, and will certainly make trials of it as soon as I can procure one of the new type. Pluitographic Copyrigld at the Berne Congress.—The Berne Congress, while occupying itself with International literary and artistic copyrights, neglected, in the first instance, to take any heed of that of photography, and it is only in the final protocol that a word is said on the subject. It is really curious how long it takes people to understand the difference between photography, engraving, aud litho graphy. The Poilevin Monument.—The agreement with the con tractor for the Poitevin monument is settled, and the work is about to be commenced at St. Calais, where it will be unveiled next September. The subscriptions, as I foretold, reached the sum of 10,000 francs, a large sum considering that the name of Poitevin is not popularly known. The Minister o f Fine Arts granted 1,000 francs. LEON Vidal. PHOTO-LITHOGRAPHY AND PHOTO ZINCOGRAPHY. BY MAJOR J. WATERHOUSE, B.S.C., Assistant Surveyor-General of India. Chapter XVIII.—Photo-Lithography in Half-Tones. In the earliest days of photo-lithography constant efforts were made to apply it to the reproduction of photographs from nature and other subjects showing gradation ot tone. Many of these efforts were more or less successful, but none were so thoroughly satisfactory as to be brought into extensive use for ordinary commercial work. At the Paris Exhibition of 1868, several excellent examples were shown by various English and Continental exhibitors. The introduction of the Woodburytype and photo collotype processes, in which the delicacy of the photo graphic gradation of tone is perfectly preserved, drew attention away from photo-lithography, and lessened the necessity for processes in which the gradation must be broken up, and the beauty of the picture more or less damaged. Consequently, of late years, little attention has been paid to photo-lithography in half tones, though it has many conveniences and advantages to recommend it; but within the last year or two the demand for cheap photographic illustrations has led to a reconsideration of the subject, principally with reference to the production of half-tone blocks to be printed from type, and several methods of block printing in half-tones have heen brought forward. Many of these methode are also applicable to photo-lithography, and therefore it seems desirable to show what has already been done in this way, and to indicate the direction in which further progress may be made. The prime difficulty in the reproduction of half-tone subjects by photo-lithography is the necessity for breaking up the continuous photographic image into a more or less strongly granular one. The same necessity exists in the photo-collotype and heliogravure processes ; but in these the grain is so fine as to ba almost imperceptible, and consequently the gradation of shade in such prints is practically continuous. In the collotype processes the amount of ink taken up by the images is in exact propor tion to the action of light, whereby the lighter shades absorb water and refuse ink, while the darker shades take up ink in proportion as they are unabsorbent of water, and thus the image is really formed of different thicknesses of ink, the slight grain tending to preserve the proper gradation. In the same way, in a heliogravure plate, the gradation of shade is represented by hollows of varying depth, and the printed image is formed by different thick nesses of ink corresponding to the different depths of the plate. The object of the grain is not to break up the image, but simply to roughen the surfaces of the hollows, so that the ink may not be wiped oat of them when cleaning the plate during the printing, but will be retained in just the proper quantity. In the case of the photo-lithographic image in half-tone, it is quite different. There is no selective taking up ot ink, the roller giving it off equally all over the image, and unless the latter is properly broken up to allow each part to print with its proper vigour and effect, the whole will soon become a black smudge. In ordinary lithography the appearance of .half tone and gradation of shade is obtained by graining the stone, and so giving it a rough surface, which breaks up the strokes of the crayon, the draughtsman getting the effect of dark or light shades by more or less pressure of the crayon on the stone, by which the little hollows forming the grain are, to a greater or less extent, filled up. Thus, although the ink roller, in passing over the drawing, leaves a layer of ink of uniform thickness and blackness all over it, the effect of pale ink in the lights is produced by the com paratively large amount of white in these parts, and the smallness and separation of the black particles of ink. In the darker parts the particles are larger and closer
