The Wonder Book Of Knowledge Part 50

Photo-engravings are a comparatively modern product. They superseded wood engravings, which for years has been the recognized medium for ill.u.s.trations to print on a type printing press. Photo-engravings, broadly speaking, are divided into two cla.s.ses--line engravings and halftones. The distinction between them lying in the fact that one, as its name implies, is a reproduction of a drawing made in lines or stipples, while the other, the halftone, gets its name from the method of its manufacture.

Briefly stated, the process of making halftones is as follows: The subject to be engraved is photographed through a halftone screen, so-called. This halftone screen is a gla.s.s plate ruled with lines at right angles ranging, for different purposes, from 60 to 200 lines to the inch. This screen is placed between the lens and the sensitized plate which is to be the negative. The necessity for this screen is due to the fact that a photograph is made up of "tones." That is to say, that the color changes imperceptibly in subtle gradations of light and shade. If this copy were photographed on a piece of copper it would present no chance for the etching fluid to act. The idea is to break up the surface into various sized dots, as the various gradations of color on the original cannot be transferred by any other method to a sheet of copper and etched.

[Ill.u.s.tration: HALFTONE ENGRAVING]

The various tones must be changed either to lines or dots, so as to make a printing surface for the ink roller of the press to operate. This is necessary to get the desired printing surface.

The dots are of various sizes, ranging from a minute stipple to a solid black, and they present to the eye the same effect as the unbroken tones of a photograph. The negative when finished shows the drawing exactly like the original. The whites are opaque, the solid blacks are clear gla.s.s, the intermediate tones showing the same values in stipples of various sizes. The film of the negative is next removed from the gla.s.s, turned and placed on a heavier plate gla.s.s with a number of others and printed on a sheet of metal which has been coated with a sensitized solution.

This plate of heavy gla.s.s containing the several negatives is placed with the sensitized metal in a printing frame. The light pa.s.ses through the clear part of the negative, the solid parts prevent the pa.s.sage of light; thus we have the light acting chemically on the sensitized surface.

After the print is removed from the printing frame, it is developed, the parts acted on by the light adhering to the metal. The opaque parts, through which no light has penetrated, leave the solution soft on the surface of the metal. This is removed by placing in water and wiping gently with absorbent cotton. The print is then dried and heated over a stove which bakes the sensitized solution to the metal. It can readily be seen that this sheet of metal is now in such shape that the etching fluid will etch away the uncovered portions of the metal and allow the protected parts, which represent the color of the original, to remain in relief.

[Ill.u.s.tration: LINE ENGRAVING]

This plate is etched--a flat proof, so called, is pulled on a hand press--and it is then taken up by the re-etcher. The re-etcher is the artist of the etching room. He takes the plate and by covering up certain parts and etching again gives additional play of color. Smaller developments of lights are worked out by careful manipulation of the etching fluid with small sable brushes. The differences in cost in the production of halftones is due largely to the length of time devoted to this work. The engraver or finisher then takes charge of it, preparing the engraving for the routing department, where the superfluous metal is removed. The plate is then returned to the engraving department, which completes the work, burnis.h.i.+ng darks, engraving highlights, removing slight imperfections and otherwise perfecting the plate.

It is then proofed and blocked. Nine separate men handle each engraving in the halftone department.

The making of line engravings follows the same general course, with the exception that no halftone screen is needed, the copy to be reproduced being already made up of lines or dots or a combination of them. In the handling of line work, eight skilled men successively handle each plate.

[Ill.u.s.tration: COMBINATION ENGRAVING]

In addition to plates made by either line or halftone process, combinations of the two are frequently used, as, for instance, where decorative pen work is used to embellish a halftone picture, or where lettering is to be used in connection with a halftone and form part of the same plate. These plates made up of both line work and halftones are known as combination plates or double-prints, depending upon the way they are produced. In both cases, negatives are made of both the halftone and fine copies.

Combination plates are made by combining the halftone and line negatives together and making one complete print on the metal.

Double-print is used where the surface is covered with halftone screen, either the line or halftone negative is printed on the metal, the other is superimposed on it.

The Benday process, so called, is the use of mechanical appliances for adding lines or stipples to either drawings or plates. Its use is very extensive in the making of tint blocks or color work, used either in connection with line or halftone key plates.

The highlight process, possible only with certain kinds of copy, is a modification of the halftone in which, by manipulation of the time of exposure and the screen when making the negative, the halftone stipples are lost and in this way halftones are produced in which there are pure whites, without the necessity of the finisher cutting them by hand.

[Ill.u.s.tration: BENDAY ENGRAVING]

Color Engravings.

Let us a.s.sume that we have a painting or a drawing in colors from which it is desired to produce a set of printing plates to produce that drawing in facsimile. Under the old method of procedure, lithography, it would have been necessary to make a stone for each of the colors, which would mean, roughly speaking, from twelve to eighteen stones to reproduce it--it will be understood that this means the finished print must go through the press once for each color. This would mean twelve to eighteen impressions to get the desired result. The expense of doing this limited the use of lithography.

[Ill.u.s.tration: HIGHLIGHT ENGRAVING]

The modern or photo-engraving method of reproducing a colored copy is based on the theory of the three primary colors, yellow, red and blue.

It is a.s.sumed that every color is formed by some combination of these three colors--the problem confronting us, therefore, is to separate these three colors and if possible make a printing plate of each color with the color values varying from light to dark in such proportions that when the three are printed in proper register over each other, with transparent printing inks, the varying color values will blend so as to reproduce the original. We go about this by making three negatives, one of each color, the red negative is made by placing at the lens a so-called color filter, which separates the red rays, whether they appear as pure red or any part of an orange or a purple, or any of the many tones of which red may form a part. In like manner the yellow and blue plates are made by the use of appropriate color filters, each of which acts for its required color as that used for the red.

So far this would appear to be a purely mechanical operation, requiring simply the usual care in negative making, but unfortunately this theory does not work out so absolutely in practice, and for this reason, while any color may be produced in light rays by the union of the three primary colors of the proper quality, when the operation is attempted with material pigments or ink, produce results varying widely from the ideal. No pigment is absolutely pure, the adulterants or foreign substances will cause sufficient deviation from the abstract standard to cause a very noticeable difference in the finished result when united with another color which is of itself impure. The result is that the three negatives, instead of each being a true unit, ready for combination with the others, is really only a basis for further work. It might justly be compared with a sketch which is all right as far as it goes, but which requires toning down and elaboration before becoming a finished work of art.

The three negatives are each printed on sensitized copper, as was noted with the black and white halftone; they are then turned over to the re-etcher, who may be rightly termed an "artist-etcher." He has before him three prints on copper; on each of them are tones which to his trained eye are too light or too dark to produce the desired result when printed with the other two, which also vary more or less. It is his duty to strengthen and reduce and otherwise manipulate the plates so that they will, when finally printed, have the desired result.

For every particular use to which an engraving can be put, there is some particular style or grade of engraving better adapted than any other.

The successful use of halftones, whether in black and white or in colors, depends on the care with which the particular screen is selected to suit the paper stock and printing conditions. To ill.u.s.trate this, the 150-line screen has 22,500 stipples to the square inch. It is apparent, therefore, that only certain kinds of paper can be used for such halftones, whereas a 60-line screen contains only 3,600 stipples to the square inch, which permits its use on a newspaper stock.

The production of engravings is just as highly technical and scientific and involves as much experience and judgment in their application as any of the learned professions.

Where are Milk-Pails Filled from Trees?

In South America there are some trees known as "cow-trees" which, when wounded, yield a rich, milky, nutritious juice in such abundance as to render it an important article of food. This fluid resembles in appearance and quality the milk of the cow.

The cow-tree is a member of the bread-fruit family, and is most common in Venezuela, growing to the height of a hundred feet. The leaves are leathery, about a foot long and three or four inches broad.

In British Guiana the name is given to another large, much-branched tree, and there are also other varieties in Para and along the Rio Negro, which is a tributary of the Amazon River.

How did the Wearing of a Crown Originate?

When we speak of a crown now we mean the head-dress worn by royal personages as a badge of sovereignty, but it was formerly used to include the wreaths or garlands worn by the ancients upon special occasions.

Among the Greeks and Romans, crowns made of gra.s.s, flowers, twigs of laurel, oak, olive and so forth, and later of gold, were made use of as honors in athletic contests, as rewards for military valor, and at feasts, funerals and so forth.

It is, however, with the eastern diadem rather than with the cla.s.sic corona that the crown, as a symbol of royalty, is connected; indeed, it was only introduced as such a symbol by Alexander the Great, who followed the Persian usage. Antony wore a crown in Egypt, and the Roman emperors also wore crowns of various forms, from the plain golden fillet to the radiated or rayed crown.

[Ill.u.s.tration: CROWNS

1. Crown of England. 2. Russian Crown. 3. French Crown. 4. Austrian Crown. 5. Imperial Crown (Charlemagne's).]

In modern states they were also of various forms until heralds devised a regular series to mark the grades of rank from the imperial crown to the baron's coronet.

The English crown has been gradually built up from the plain circlet with four trefoil heads worn by William the Conqueror. This form was elaborated and jeweled, and finally arched in with jeweled bands surmounted by the cross and scepter. As at present existing, the crown of England is a gold circle, adorned with pearls and precious stones, having alternately four Maltese crosses and four fleur-de-lis. From the top of the crosses rise imperial arches, closing under a mound and cross. The whole covers a crimson velvet cap with an ermine border.

The crown of Charlemagne, which is preserved in the imperial treasury of Vienna, is composed of eight plates of gold, four large and four small, connected by hinges. The large plates are studded with precious stones, the front one being surmounted with a cross; the smaller ones, placed alternately with these, are ornamented with enamels representing Solomon, David, Hezekiah and Isaiah, and Christ seated between two flaming seraphim.

The Austrian crown is a sort of cleft tiara, having in the middle a semicircle of gold supporting a mound and cross; the tiara rests on a circle with pendants like those of a miter.

The royal crown of France is a circle ornamented with eight fleur-de-lis, from which rise as many quarter-circles closing under a double fleur-de-lis. The triple crown of the popes is more commonly called the tiara.

Why do Lobsters Change Colors?

Before a lobster is cooked he is green, that being the color of the rocks around which he lives on the bottom of the ocean. However, as soon as a lobster is placed in boiling water his sh.e.l.l changes from green to red. This is due to a certain chemical substance contained in the sh.e.l.l which acts in that way when boiled.

How do Fishes Swim?

The fish is entirely surrounded by water which exerts an equal pressure on all sides. When the fish moves its tail, or makes any movement at all, he moves in the water. Of course, by moving his tail from side to side he propels himself forward and by bending his tail he goes in the direction in which it is bent.