254 



KNOWLEDGE. 



[NO^-KMBEB 1, 1897. 



negative and its coating is washed away, except where the 

 insohible lines are found. In this state it is (theoretically) 

 ready to be placed in a batb of acid, which would eat away 

 the exposed portions of the metal, leaving in relief the lines 

 protected by the insoluble film, which would protect the 

 metal. Practically, it is necessary that the power of these 

 lines to resist the acid should be reinforced by coating them 

 with a fatty ink, and the application of such ink (to protect 

 the sides of the lines as the acid eats down) is usually 

 repeated several times during the etching. 



When the etching is deep enough, the metal plate is 

 cleaned from its protective coating and mounted upon a 

 wood block to raise it to the thickness of ordinary printing 

 type. 



In the half-tone process we have to face the difficulty 

 that the originals are not broken up into dot or line, but 

 consist of flat gradations of tint, as in photograms or wash 

 drawings. In this case, if a negative were made as above 

 mentioned, it would consist of dense parts, transparent parts, 

 and every possible grade of opacity between. The coating 

 on the metal plate, after printing, would have perfectly 

 soluble and perfectly insoluble parts, with every gradation 

 between, and in the washing o£f would come patchy and 

 totally unsatisfactory. Altogether, the resulting block 

 would be one from which no printer could make a print 

 fit to be seen. The overcoming of this difficulty has 

 employed the thoughts of many able men, whose efforts 

 have resulted in a satisfactory process which is now in 

 universal use. 



Essentially, the half-tone process is the same as the line 

 process just described, except in the matter of the negative. 

 In making this, the original to be reproduced is fixed on 

 its easel, and the camera is arranged very much as in the 

 line process. Immediately in front of the sensitive plate, 

 however, is placed a glass plate ruled with fine cross-lines, 

 after the fashion of a micrometer grating. This is called 

 the cross-line screen, and its action is not nearly so 

 obvious as might at first be supposed. In fact, the screen 

 was in everyday use by hundreds of firms and thousands 

 of operators for some years before its real action was 

 known. To explain that action would take more space 

 than is at my command, and would lead into somewhat 

 abstruse optical and mathematical details. Suffice it to 

 say that the practical result is that the high lights of the 

 resulting block consist of very fine dots, surrounded by 

 white ; while the shadows are represented by larger dots 

 with but small spaces between them, or even joined 

 together to form solid blacks. This effect will be seen by 

 examining any print from a half-tone block (for instance, 

 those on page 269 of this journal), and is all the more 

 clear if a magnifying glass is used. 



As it is necessary that a fine " screen" should be used, 

 to make fine dots, if the picture is to show any delicate 

 gradation and detail, it foUows that the metal cannot be 

 etched very deeply between the dots, and this shallowness 

 of the half-tone block is its great fault in the eyes of the 

 printer. It necessitates the use of a very fine stiff ink, and 

 hard unbroken inking rollers, or the fine shallow spaces 

 between the dots will fill with ink and the resulting print 

 wDl be spotty. The paper, too, must be fairly stiff, or it 

 will be pressed into the shallow spaces ; and it must not 

 be fluffy, or the surface hairs will be pulled off by the stiff 

 ink, and rolled by the inking rollers into the spaces of 

 the block. Before the introduction of these fine blocks 

 it was the custom of printers to always work with their 

 paper damp, in order that it might take the inked im- 

 pression more kindly, and with half-tone blocks this 

 " wet " printing is impossible. Curiously enough, for our 

 knowledge of the '• dry " printing method, and largely for 



the present position of illustrated journaUsm, we are 

 indebted to the American War of the Rebellion. 



The " dry " printing was really first worked out by some 

 of the army printers, in times when water and the facilities 

 for damping paper were scarce. Working against their 

 difficulties, and modifying their ink and their inking to 

 make the best of a bad job, they found that, before long, 

 they were able to make even better results with dry than 

 with damp printing. Once this was established, it became 

 possible to improve the face of the paper by rolling and 

 by other methods, giving it a smooth surface that would 

 be ruined by damp printing, but that rendered the dry 

 impression immensely more fine than any damp impression. 

 Before long, dry printing and the highly faced papers were 

 in use in the States for all fine work ; and when half-tone 

 began to be used, the American printers, as a whole, were 

 much better prepared to tackle it than were the British, 

 who had less generally adopted dry printing. Hence, the 

 American half-tone printing, and the American manufac- 

 tui-e of papers and inks for the work, have been, until 

 quite recently, far ahead of anything European. 



I have dwelt at some length on these relief-printing 

 processes because of their great importance, and have now 

 only space for the briefest possible description of collotype 

 printing,* with which, however, photo-lithography is closely 

 allied. In this process a negative is used, somewhat as 

 before, but the surface for eventually printing from is 

 plate glass (usually) coated with bichromated gelatine. 

 After this has been exposed under the negative, the gelatine 

 which remains soluble is not removed, but simply has the 

 bichromate soaked out of it. The plate then remains 

 covered with gelatine, part of which (the whites of the 

 picture) is water absorbent, while the rest, for printing 

 the shadows, is hard and dry. An inking roller charged 

 with greasy ink is passed over the whole surface, and, 

 since the grease is repelled by water, the ink only adheres 

 to the dry parts of the film. The paper is brought in 

 contact with this and rim through a lithographic press, 

 and the picture appears. 



It is impossible to deal with photo-aquatint, photogravure, 

 Woodburytype, and other kindred processes in the present 

 space, in which I have merely attempted an outline of the 

 leading photo-mechanical methods. My hope is that the 

 outline may be sufficiently clear to give readers some notion 

 of how their pictures are produced. 



THE FLIGHT OF A SEED. 



By Rev. Alex. S. Wilson, M.A., B.Sc. 



THE higher plants are fixed throughout life to one 

 spot ; they have no power of locomotion. Nature 

 has so far made amends by providing the seeds of 

 many species with appendages of various kinds 

 which greatly facilitate their dispersion. The hairs 

 or plumes with which so many seeds are furnished constitute 

 a very effectual appliance of this kind. Hairs, if horizontal, 

 prevent a seed from falling quickly ; if vertical, they act 

 like sails, so that the seed is impelled rapidly before the 

 wind. The presence of such appendages often entails other 

 related adaptations, very much on the principle that a man 

 who has a horse also requires harness, and in some of 

 these auxiliary contrivances nature seems to have taxed 

 her ingenuity to the utmost to secure for each seed a 

 fair chance of development. Sometimes the entire 

 outer surface of the seed is beset with long silky hairs, 

 as in the cotton-plant {Go^si/pium}. In the allied Ster- 

 cuUaceee, or silk-cotton order, several genera, including 



* See also " The Collotype Process and Photo-Engraving," bj A. 

 C. Banyard. (Knowlbdqb, February, 1890.) 



