October 13, 1904] 



NA TURE 



579 



of Abney colour sensitonieters, or by spectroscopic 

 methods, but it is not sufficient to judge of the colour 

 by the eye alone. It is necessary that the colours shall 

 not only look right, but that they shall be right when 

 analysed spectroscopically. Each colour is used, not 

 so much for its own sake, as to give the proper tints 

 w hen the light it reflects or transmits is mixed with 

 the light from the other colours. 



But supposing that the inks or stains selected were 

 theoretically perfect in colour, and perfectly trans- 

 parent, so that when superposed the lowest produced 

 its full effect, it still remains to see that each is laid on 

 in its due proportion, for if one is in error the whole 

 will be thrown out, and what should be neutral greys 

 will be tinted with colour. And there is still another 

 difficulty to consider. The gradation produced by the 

 same treatment of a sensitive plate generally becomes 

 more steep as the wave-length of the light used in- 

 creases. This is certainly the rule, though there may 

 be exceptions to it. Therefore, if the pink, yellow, and 

 blue inks are properly proportioned to give a neutral 

 black where the colours are in full quantity, the greys 

 inav be expected to be bluish, and in the lighter colours 

 there may be expected a deficiency of red. To over- 

 come these difficulties, some have sought to use four, 

 or even five, colours instead of three, and others have 

 used a fourth plate with black ink to give the blacks 

 and greys, removing from the plate those parts where 

 pure colours are required. Obviously, the use of 

 additional printing plates is a confession that the three- 

 colour process cannot be worked on the theoretical 

 lines, but such variations as these do not appear to be 

 at all generally adopted. 



There are many methods by which the three colours 

 may be brought together to make the final picture. 

 The simplest of all, both theoretically and practically, 

 and at the same time perhaps the most perfect and 

 therefore the most beautiful, appears to be the pro- 

 duction of transparencies by the superposition of 

 gelatine reliefs produced from the colour records, each 

 appropriately stained by immersion in a suitable dve 

 solution. There is nothing to regulate the depth of 

 colour to which each relief should be stained but the 

 judgment of the worker, and the fact that the greys 

 and blacks of the original should be untinted by colour 

 in the reproduction. .A relief may be more highly 

 coloured by a further immersion in the dye solution, 

 or lightened by immersion in water. 



For printing on paper, either the collotype process, 

 lithography, or typographic blocks may be employed. 

 Some very excellent work has been done by the first 

 of these methods, but in both collotype and lithography, 

 as already stated, it is difficult to regulate, to a nicety, 

 the amount of ink applied, and the successful im- 

 pressions generally form only a small proportion of the 

 whole. The process that is the most used, and that 

 certainly at the present is the most suitable, consists 

 in printing from typographic blocks as ordinary letter- 

 press is printed. The photographic production of 

 blocks from half-tone negatives is practised to so 

 great an extent, and the machinery for letterpress 

 printing is so perfect, that uniformity is more easily 

 secured by this than by other methods. In pointing out 

 the advantages of this process. Colonel Hiibl states 

 that it gives a better blending of the colours, and that 

 the inks are not brought into contact with water so 

 that they remain pure, and as the paper remains dry 

 during the printing a better register is secured. 

 But the process has its own defects — Hiibl says that 

 "the gradation is limited and incorrect," and that 

 " the final result depends more on the clever work of 

 the retoucher than on the perfection of the photographic 

 negative." This last statement conveys, perhaps, 

 rather an exaggerated view of the facts of the case. 



NO. 1824, VOL. 70] 



The dots in the impression from a half-tone block 

 are, as is well known, in lines, as determined by the 

 ruling of the screen that is placed in front of the plate 

 when the grained negative is made. Now when lines 

 at regular intervals cross each other at a small angle 

 a pattern is produced known as a " moire " or " water- 

 ing " effect, from its similarity to the appearance of 

 " watered " silk. The effect can easily be seen by 

 looking through two pieces of muslin superposed with 

 the threads not quite parallel. Two rows of park rail- 

 ings, if near enough, will often show it, the railings 

 in each row being parallel to each other, but not quite 

 parallel to those in the other row. In superposing 

 three impressions from blocks made by means of cross- 

 lined screens, it is important to avoid any approach to 

 parallelism of any set of lines or rows of dots with any 

 other, especially as regards the relative positions of 

 the red and blue impressions, as these are darker than 

 the yellow, and therefore more conspicuous. It is 

 obvious that if the screen used has its crossing lines 

 at right angles, the three impressions will give six sets 

 of lines, and an angle of 30° between each and the 

 next if the angles of crossing are kept equal. Circular 

 lined screens mounted in aluminium cells, so that they 

 can be rotated to any desired angle with precision, are 

 much used in America and are coming into use here. 

 .\ common method, which is being superseded , by 

 the circular screen, is to use two rectangular 

 screens, one of which is ruled at such angles 

 that, by turning it round from back to front, it gives 

 the rulings in the third position. Other methods 

 are sometimes used for the purpose, for the shape of 

 the aperture in the diaphragm affects the character of 

 the dots, and it is possible to use a slit diaphragm and 

 rotate the lens that carries it. But the most notable 

 variation in the preparation of the blocks is to get rid 

 of the lines, or even rows of dots, altogether, by using 

 an irregular grained screen. Of the many attempts 

 that haVe been made in this direction, the one that 

 seems to offer the greatest promise of success is 

 Wheeler's " metzograph " screen, in which an 

 irregular wavy surface of the glass gives the concen- 

 trations of the light required. 



There is one matter in connection with three-colour 

 printing by means of half-tone blocks that some- 

 times presents a difficulty to "the minds of those who 

 have a slight acquaintance with the subject, and 

 certainly is not altogether without effect, namely, the 

 fact that the dots of the different colours are sometimes 

 superposed and sometimes side by side, according to 

 accident in printing. This difference is not very 

 important. The colours are arranged on the sup- 

 position that they are superposed. If otherwise, much 

 the same tint results, but it is mixed with white, be- 

 cause the result is the sum of the lights instead of the 

 sum of the absorptions. But if the dots were com- 

 pletely superposed, then there would be a greater area 

 of wliite paper between the dots, and the white light 

 from this source doubtless about compensates for the 

 other. Perhaps, in the circumstances, the juxta- 

 position is advantageous, as the tendency for the ink 

 last applied to show more conspicuously than the 

 others would not hold in this case. 



.A notable example of the excellence that is now 

 possible bv three-colour typographic printing may be 

 seen in "The Water-Colour Drawings of J. M. VV. 

 Turner, R..A.," a selection of fifty-eight subjects, pub- 

 lished bv Messrs. Cassell and Company, the reproduc- 

 tions being made by Messrs. Andr^ and Sleigh at their 

 works at Bushey. ' In such a case the question that 

 is naturally asked is, Are the colours of the reproduc- 

 tions similar to those of the original? Of course 

 they are not the same pigments — this is not in- 

 tended ; the only aim of the printer is to give colours 



