November, 1905.] 



KNOWLEDGE & SCIENTIFIC NEWS. 



279 



Photography. 



Pure and Applied. 



By Chapman Jones, F.I.C, F.C.S., &c, 



Pinaiype. —This is the latest method of three-colour 

 photography, the instructions and materials for which 

 are issued by the colour works, formerly Meister, 

 Lucius, and Briininp. Messrs. Fuerst Bros., of Phil- 

 pot Lane, are the English agents. The method is, I 

 believe, intended for the use of amateurs and those 

 who make their own prints rather than for commercial 

 work. The ordinary three negatives are made through 

 a blue, green, and red colour-screen respectively. From 

 each of these a transparency is made either by super- 

 position or by common enlarging methods, and each 

 transparency in turn furnishes a " print plate " by 

 exposing beneath it a plate coated with gelatine and 

 rendered sensitive by means of potassium bichromate. 

 After due exposure the " print-plates " are washed, 

 and, with or without drying, each is placed in a solution 

 of the appropriate dye. The colouring matter is not 

 absorbed where the gelatine has been completely 

 hardened by the exposure, but it is taken up in the 

 other parts, most copiously where there has been least 

 exposure under the positive. The final print is pre- 

 pared by squeegeeing a piece of " transfer paper " 

 (gelatine coated) on to each " print-plate " in turn, 

 with due regard to register, condition of moistness, and 

 time that it is allowed to remain on the "print-plate," so 

 that the amount of each dye absorbed by the " transfer 

 paper " may be duly proportioned. The " print- 

 plates " can be used as often as desired, preferably 

 soaking them for a few minutes, each in its proper dye, 

 solution, for each final impression. The resulting 

 prints are finally placed for a minute or two in a solu- 

 tion, the formula of which is not stated, " to increase 

 the pictures' fastness to light " and harden the gela- 

 tine, washed for about five minutes, and hung up to 

 dry. The method is also applicable to the preparation 

 of coloured transparencies. 



T/ie Royal Fhotographic Society's Exhibition. — As the 

 Exhibition will be closed before this journal is pub- 

 lished, my notes on it must be purely retrospective. 

 It is vcrv satisfactory to find that the practice of record- 

 ing changes by photography is increasing. Not merely 

 recording the phases of rapid motion, as in a series of 

 photographs of a splash, but slower movements, as in 

 the skin moult of a caterpillar, the germination of a 

 prass seed, and the difference caused in an oak tree by 

 fifty years of life. A mere photograph of a living 

 creature is now common-place, unless the animal is 

 rare; to secure attention some characteristic act or 

 attitude must be shown, such as a flock of gulls alight- 

 ing on the water, a robin eating a worm, or swans 

 flving in anger at their assailant. A set of sixty 

 lantern slides of mediaeval baptismal fonts was medalled 

 by the judges, because such work shows considerable 

 sacrifice in the attainment of a specific object, and an 

 object worthy of the sacrifice. A set of lantern slides 

 should always have a definite aim, and if the aim is not 

 obvious it should be stated by the exhibitor. The 

 " pinatype " method of colour photography was demon- 

 strated at the Exhibition. 



Paper as used iti Photography.— Vor certain photo- 



graphic purposes, as in printing processes where the 

 sensitive substance comes into immediate contact with 

 the paper, and is, in a measure, absorbed by it, as in 

 platinum and albumenised silver paper, it is necessary 

 to have paper prepared with great care from pure 

 materials. But when the sensitive material is prepared 

 as an emulsion and applied so as to form a distinct 

 layer, as in the usual printing out papers (P. O. P.), it 

 is desirable to have a srnoother and less absorbent sur- 

 face than that of simple paper. For such purposes the 

 paper receives a preliminary coating or enamelling, as 

 of gelatine hardened with chrome alum, and coloured 

 with a pigment, such as madder lake, or whitened and 

 made more opaque by a white pigment, such as barium 

 sulphate. But in order to get the most perfect im- 

 pressions from half-tone typographic blocks with their 

 minute and, therefore, shallow etched dots, a still more 

 perfect surface is necessary, and this has led to the 

 introduction of " art papers." From a communication 

 made by Mr. R. W. Sindall to the Society of Chemical 

 Industry and the discussion that followed it, we learn 

 that " art papers " are coated on both sides with some 

 inert mineral substance, such as china clay, barium 

 sulphate and alumina, slaked lime and alum, or pre- 

 cipitated calcium sulphate, which is mixed with a 

 sufficient quantity of an adhesive, such as glue or 

 casein, to hold it together during the printing process 

 without offering a too little absorbent surface to the 

 ink. A coloured pigment is added if desired. Casein is 

 largely used, and as it is insoluble in water, borax or 

 ammonia is added with it to dissolve it, and a little 

 formaline may be added also to get rid of any smell due 

 to incipient decomposition. A heavy " art paper " 

 may contain about 35 per cent, of mineral matter, and 

 the thickness of the original will be increased by some 

 12 to 15 per cent, by the coatings. 



A pure paper must present a very uneven surface, be- 

 cause of the irregularly overlaying fibres. The addi- 

 tion to the pulp of from 5 to 10 per cent, of mineral 

 matter gives a rather smoother surface, and twice this 

 amount gives a marked improvement. A so-called 

 " imitation art paper " is better still and may contain 

 30 per cent, of mineral matter, though it is not applied 

 as separate coatings as in the best " art papers," but 

 put into the pulp, and brought to the surface to a 

 certain extent by moistening the paper just before it is 

 drawn through the rolls of the calender. 



I suppose that an impression in black printer's ink 

 is one of the most permanent kinds of record that can 

 be imagined, but obviously its life is limited by the 

 durability of the substance that it rests on. No doubt 

 there is a tendency to consider that when the paper 

 proper is to be sandwiched between two surfacing 

 layers, it is not necessary to pay very much attention to 

 its quality. But however this mav be, it is certain that 

 from a chemical and often from a physical point of 

 view every increase in complexity means an added 

 possibility of disintegration or decomposition. Gela- 

 tine (or glue) and casein are particularly susceptible to 

 damp. In " art papers " the impression does not lie 

 on paper at all but upon the surface of the coating. 

 The preparation of a paper that shall satisfy the de- 

 mands of the printer and at the same time be free from 

 suspicion when regarded from its chemical and physi- 

 cal aspects, remains a very difficult if not an unsolved 

 problem, but it is well to know how we really stand in 

 this matter, that we may not regard only the permanent 

 character of the impressions of our " reproduced " 

 photographs, but also the resisting power to adverse 

 influences of the material on which they are made. 



