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poses, and to give a summary of the principal methods that may be usefully employed with reference to the wants of the State or of private individuals, rather than to those of professional cartographers and map-publishers, though the latter may in many cases also find photography a useful auxiliary. Photographic methods can never entirely take the place of lithography or engraving by hand, either for public or private purposes, but their use may be advantageously extended. Those who wish for fuller details may consult the text-books by Abney, Carey Lea, Monckhoven, Vogel and others, and the special works referred to in this paper.
PREPARATION OF THE ORIGINAL DRAWING.
I have already adverted to the difficulty that has been found in this country and elsewhere in obtaining original drawings suitable for reproduction by photozincography, and to the fact that without a proper original drawing it is quite impossible to produce satisfactory results. Besides its principal use in reproducing maps of the Surveys, photozincography is very largely utilised in India by engineers for the reproduction of their plans and drawings, and by other public officers for an immense variety of miscellaneous maps and plans, and as we were constantly asked to photozincograph subjects utterly unsuitable to the process, a set of rules for the preparation of the original drawings for reproduction by photozincography was drawn up under General Thuillier's direction and published in the official Gazettes all over India, and the result has been a great improvement in the execution of the drawings we receive for reproduction.
The rules are as follows:
1. All drawings should be on white, smooth-surfaced paper, free from dirt, pencil marks, creases and wrinkles. When possible they should remain stretched on the drawing-board.
2. The Indian ink should be freshly rubbed down and give good black lines, free from glaze.
3. The lines should be firm and cleanly drawn—not too fine or too close together. They must be quite black, and light effects must be produced by fine and open black lines, and never by the use of pale ink. Thick lines in the printing and borders of maps should be well filled in. Pencil marks should be carefully removed, so as not to injure the blackness and firmness of the lines.
4. All cross-hatching and shading should be as open and clear as possible, and the lines composing it firm and not too fine. Intensity of shade must be shown rather by an increase in the thickness of the lines than by placing them closer together, in order that the intermediate spaces may not become blocked up when transferred to zinc. It is better not to rule the shading of mechanical and architectural section-drawings, but to show the shaded parts by a light tint of blue, violet, or aniline red (fuschine or roseine). These parts will reproduce white, and can have a ruled tint transferred on the stone or zinc in the usual way, which will give a much neater appearance.
5. In plans or drawings intended for photozincography, washes of any colour except very pale blue, violet, or aniline red, are absolutely inadmissible. Outlines, may, however, be drawn, if necessary, in any strong red, brown, yellow, orange or green pigment which will reproduce black. Any details required to be shown in the original, but not in the copy, may be drawn in pale blue, violet, or aniline red. Details that are not required to be reproduced may be painted out with Chinese white.
6. River courses, lakes and tanks should be left blank, and not filled in with fine lines. They may be indicated by a pale wash of blue without detriment to their reproduction.
7. When drawings are to be reduced care must be taken to draw the lines, lettering, and detail of sufficient thickness and size relatively to the scale of reduction, so that they may not be lost or illegible when reduced. Sufficient space must also be left between the lines to prevent subsequent blocking up.
8. When possible, drawings should be made on a larger scale than they are required to be copied. Photographic reductions are always sharper and firmer than reproductions to the same scale, and defects in drawing are lessened by reduction.
9. Where plans or drawings to scale are to be reduced, the scale should be given in terms of a single unit of measurement and not as relative to any second unit. Thus, the scale on a map drawn on the scale of 4 miles to an inch for reduction to 16 miles to an inch, should be shown simply as a 6 scale of miles.”
10. As photography produces a more or less perfect facsimile of the original drawing, it is essential that drawings intended for publication should be complete and finished in every respect before they are made over to the photographer. The drawing, printing of names, &c., should be in as neat a style as possible, and not require to be altered or touched up. The hair-strokes of the printing should not be too fine.
The foregoing rules may be summed up in a few words :-WHITE-PAPER, BLACK-INK, and TIRM OPEN DRAWING ; and as success in the after processes depends entirely upon the perfection of the original drawing and its capability of giving a negative on which the ground is perfectly opaque while the lines are quite clear and as transparent as the bare glass, these essentials must be most carefully observed. Their neglect will entail failure and disappointment.
For drawings intended for reproduction by the collotype methods these
rules are equally applicable, especially No. 7, and there is even more necessity for perfect cleanliness of the paper and neatness and finish of the drawing, because the faintest tints will be reproduced by the gelatine printing surface and corrections cannot be made on it, as they can on zinc, stone or copper. For this reason also, the greatest care must be taken to complete the drawing in every respect before it is given to be reproduced. Drawings in line may be finer and more delicate than for photolithography, but still must not be so fine as to interfere with the obtaining of a perfectly dense and opaque negative, otherwise the ground of the print will appear dirty and stained. Pale ink may be used when necessary for effect, but not more than is really requisite. Colour may be used to any extent, having always due regard to the photographic effect when reproduced. On account of the difficulty of photographing certain colours so as to produce the same effect as in the original picture, the best results will be produced from drawings specially prepared in monochrome, such as Indian ink or sepia.
In the case of drawings for any special purpose or not intended for publication, the above rules may be relaxed, but the general principles laid down should be observed, as far as practicable, if the best results are desired.
When drawings are prepared specially for photographic reproduction, there need be no difficulty in taking all the precautions necessary for producing good results. It often happens, however, that the photographer is called upon to reproduce drawings, lithographs, or old MSS., printed records, or engravings, which either may never have been suitable for the purpose, or, if suitable when fresh, have become dirty and stained by age. Herr Scamoni, the skilful Chief of the Photographic Department of the Imperial State Paper Office at St. Petersburg, has given some useful hints on the treatment of such subjects under these circumstances.*
“ Yellow, or otherwise objectionable, spots should be carefully covered over in the spaces between the lines with Chinese white, and whenever possible the lines should be strengthened in parts where they appear weak.”
“ Lithographs and engravings may be bleached, by immersion in a solution of chloride of lime, or Eau de Javelle, (1 to 10 or 15 of water), then soaked in water for some hours, after which they are treated with a weak solution of hyposulphite of soda and finally well rinsed in clean water."
“Fresh grease stains may be removed with chloroform, benzine and ether, or with a weak alkaline solution of caustic potash or its carbonate.”
“Old grease stains may be removed with a more or less strong solution of potash, applied at the back of the subject.”
* Handbuch der Heliographie, p. 67.
“Iron mould and ink spots may be taken out with a solution of oxalic acid or salts of sorrel.”
When tracings are made on paper or vellum cloth to be reproduced without the aid of the camera, special care must be taken to keep the back of the drawing clean, and to choose paper or cloth free from stains and of as even a texture as possible.
Originals drawn on rough paper may be smoothed in a copper plate press, and, if dirty, should be carefully cleaned with india-rubber or bread.
III. THE PRODUCTION OF THE NEGATIVE.
After the due preparation of the original, the production of the negative is a point of the utmost importance, and may well be considered by itself before proceeding to the consideration of the various processes of photographic printing.
In order to obtain the most satisfactory results for photolithography, photozincography, or any other process specially applicable to line subjects, the negative must be perfectly sharp all over, free from distortion and possess the greatest amount of contrast between the lines and the ground. If care is taken to produce good negatives from suitable originals, results may
be obtained which will compare with ordinary lithographs and engravings for sharpness and delicacy. The difference in the results of working with good negatives or bad ones is incredible ; with a good negative from a good original every thing works well, but with a bad negative from a faulty original all kinds of difficulties may be encountered, and the attainment of a passable result is almost a matter of chance.
The first thing is to arrange the plan so that it may be copied without any distortion and be quite sharp all over.
To ensure freedom from distortion, the lens employed must give an image quite free from all curvature of the marginal lines of a rectangle. In practice the most suitable forms have been found to be the ‘Rectilinear? of Dallmeyer; the 'Doublet' of Ross ; ' Aplanatic' of Steinheil and others on the same principle. The lenses known as triple combinations are also good. In the Surveyor General's Office, Calcutta, Dallmeyer's Rapid Rectilinears are used and found to answer well. The lens should be worked well within its power, so as to use the most central rays; and to secure the sharpness of the image all over the plate, a small stop or diaphragm should be used.
The plan must be placed so as to be evenly illuminated by a good strong light falling as horizontally as possible, in order to avoid shadows being thrown by the grain of the paper, and thus diminishing the even opacity of the ground of the negative.
The apparatus for supporting the plans varies according to the nature of the work required, and may either be a perfectly smooth board fixed permanently in a truly vertical position against a wall or other support, a form which is very suitable when large plans have to be copied or reduced ; or it may consist of a frame large enough to take a certain size of map and capable of being adjusted in various ways so as to move up and down in a vertical plane or horizontally right and left, so that different parts of the plan may be brought in front of the camera without moving the plan on the board.* In any case, arrangements must exist, either in the plan-board or in the camera-stand, for making the plane of the map or plan to be copied exactly parallel to the plane of the sensitive plate in the camera.
The map must be attached to the plan-board so that it may lie perfectly flat and free from ridges. This is best secured by placing in front of it a sheet of glass which is fastened down on the board with pins at the corners. Or a glazed frame may be used for holding plans of a medium size. In either of these cases care must be taken to avoid
reflection from light objects in front of the plan-board.
It is convenient to have the plan-board and the focussing glass of the camera ruled in squares of 1 inch or other convenient size, in order to at once test the perfect parallelism of the sensitive plate and the plan-board.
When the work is confined to the reproduction or reduction of maps or other subjects of one fixed size on a single plate, it will be found convenient to draw a rectangle of the required size on the ground glass of the camera. When the image of the subject exactly fills this rectangle the adjustments of focus and parallelism will be correct.
The camera used for reproduction to scale should be at least of sufficient length to draw out to twice the equivalent focal length of the largest lens it is to be used with, and may be furnished with cone fronts to give further extension if necessary. With large cameras of a long range of focus it will be found convenient to have the back part of the camera fixed and the front part carrying the lens moveable, so as to enable the operator to focus conveniently. The camera may be fixed on a stand furnished with adjustments for moving it horizontally right or left, and have a tilting motion up and down, in order to adjust the camera perfectly level, or tilt it slightly so as to correct any want of verticality of the plan-board. The camera-stand should run upon rails fixed in the ground at right angles to the wall carrying the plan-board, thus enabling the distance of the camera from the plan-board to be easily and accurately adjusted according to the scale required. When using a reversing mirror or prism for taking re
* See my ' Report on the Cartographic Applications of Photography,' plates V, VII and X, and Sir H. James'' Photozincography', plates I and II.