?26 



SCIENTIFIC NEWS. 



[April 6, 1 5 



of -the drawing in insoluble gelatine, because wherever 

 the light has penetrated the clear parts or lines of the 

 negative it has, through the addition of the bichromate 

 of potash so oxidised and changed the nature of the 

 gelatine that it cannot again be dissolved even in hot 

 water. But there is yet another valuable property to be 

 mentioned in connection with this oxidised gelatine, and 

 that is its capability of taking printing-ink. Suppose I 

 take this exposed piece of gelatinised paper, acted on 

 by light as mentioned, and soak it for a few minutes in 

 cold water, then roughly drying it with blotting-paper, 

 lay it on a flat board, and, taking a roller charged with 

 printers' ink, pass it gently over two or three times, I 

 shall find that the exposed lines will, greedily take ink 

 from the roller, whilst the unexposed or white parts will 

 refuse the slightest particle. I shall now have a 

 facsimile copy of the lines of my negative in printers' 

 ink on the gelatinised piece of paper, and if the ink I 

 used were what is known as transfer ink, the inked 

 piece of paper would be a photo-litho transfer, and ready 

 as soon as dry for laying down upon a lithographic stone 

 for printing from in the ordinary way, or, if desired, a 

 plate of zinc might be substituted for the lithographic 

 stone, and by passing the zinc plate through a press in 

 contact with the transfer, the greasy ink would leave the 

 gelatine and attach itself to the zinc, and giving a plate 

 ready for the photo-zinc etcher to make into a typo- 

 graphic block. 



Having thus roughly sketched the general principle 

 of the foundation of nearlj' all photo-mechanical printing, 

 I will briefly discuss the processes referred to, and 

 endeavour to describe the working. 



As I remarked, the most important process, owing to 

 its simplicity and cheapness, is that of photo-zinc etched 

 blocks for ordinary typographic printing. I can, of 

 course, only simply state the generally employed method 

 of producing these blocks, and slightly touch upon the 

 other subjects within the limits of this paper. For 

 exact formulas and details for successfully working any 

 one of these processes, I must refer you to specialists 

 who make these arts a life study, and to the few works 

 at present published on the subjects. 



First, then, the operations ot etching a type block, say 

 from a line drawing. 



The drawing intended for reproduction is pinned on 

 a board and placed squarely before a copying-camera in 

 a good even light. The lens used for this purpose must 

 be capable of giving a perfectly sharp picture right up to 

 the edges, and must be of the class called rectilinear, 

 i.e., giving straight lines. The picture is then accurately 

 focussed and brought to the required size ; a plate is 

 prepared in the dark room by the collodion process, 

 which is then exposed in the camera for the proper time 

 and developed in the ordinary way ; after development, 

 the plate is fixed and strongly intensified in order to 

 render the white portions of the drawing as opaque as 

 possible. On looking through a properly-treated negative 

 or this kind it will be seen that the parts representing 

 the lines and black portions of the drawing are clear 

 glass, and the whites representing the paper a dense 

 black. The negatis'e after drying is ready for the next 

 operation, i.e., printing upon zinc. This is done in 

 several ways. One method will, however, be sufficient 

 for the purpose here. I obtain a piece of the bichroma- 

 tised gelatine paper previously mentioned, and place it 

 on the face of the negative in a printing-frame. This is 

 exposed to sunlight (if there be any) or daylight for a 



period varying from five to thirty minutes, according to 

 the strength of the light. This exposed piece of paper is 

 then covered all over with a thin coating of printing-ink, 

 and wetted in a bath of cold water. In a few minutes 

 the ink leaves the white or protected parts of the paper, 

 remaining only on the lines where the light has passed 

 through the negative and affected the gelatine. We now 

 have a transcript of the drawing in printing-ink on a 

 paper which, as soon as dry, is ready for laying down 

 on a piece of perfectly clean zinc, and passing through a 

 press. The effect and purpose of passing this cleaned 

 sheet of zinc through the press in contact with the picture 

 on the gelatine-paper is this : owing to the stronger at- 

 traction of the greasy ink for the clean metal than for the 

 gelatine, it leaves its original support and attaches itself 

 strongly to the zinc, giving a beautifully sharp and clean 

 impression of our original drawing in greasy ink on the 

 surface of the zinc. The zinc plate is next damped and 

 carefully rolled up with a roller charged with more 

 printing-ink, and the image is thus made strong enough 

 to resist the first etching. This etching is done in a 

 shallow bath, which is so arranged that it can be rocked 

 to and fro. For the first etching, a very weak solution of 

 nitric acid and water is used ; the plate is placed with 

 this acid solution in the bath, and steadily rocked for five 

 or ten minutes. The plate is then taken out, washed, 

 and again inked ; then it is dusted over with pow- 

 dered resin, which sticks to the ink on the plate ; after 

 this the plate is heated until the ink and resin on the 

 lines melt together and form a strong acid-resisting 

 varnish over all the work. The plate is again put into 

 the acid etching bath and further etched ; these operations 

 are repeated five or six times, until the zinc of the 

 unprotected or white part of the picture is etched deep 

 enough to allow the lines to be printed clean in a press, 

 like ordinary type or an engraved wood-block. I ought, 

 perhaps, to explain that between each etching the plate 

 is thoroughly inked, and that this ink is melted down the 

 sides of the line, so as to protect the sides as well as the 

 top from the action of the acid ; were this neglected, the 

 acid would soon eat out the lines from below. The 

 greatest skill and care is, therefore, necessary in this 

 work, especially so in the case of some of the exquisitely 

 fine blocks which are etched for some art publications. 



(To be continued.) 



»^>*^i>^5*f-» ■ 



IRIDESCENT CLOUDS. 



MR. J. C. McCONNEL, according tothePopular Science 

 Monthly, has studied the phenomena of iridescent 

 clouds at St. Moritz, in Switzerland, where they are very 

 common in winter, occurring usually whenever there are 

 scattered clouds near the sun. Within a circle of about 

 2° radius the clouds are whitish, faintly tinged with blue. 

 This space is surrounded by a ring of yellow or orange. 

 The region of most vivid hues is comprised between 

 3° and 7'', and the most striking tints are purple, orange, 

 green, and red, distributed in irregular patches over 

 the thinner parts of the clouds. Beyond this region the 

 only colours visible are green and red, becoming fainter 

 as the distance from the sun increases. He supposes 

 these colours to be the result of diffraction of light by 

 fine particles of ice. 



Waste Land in India. — In India ten million acres of 

 good land have never been cultivated, and 120 million 

 acres are returned as wastes. 



