The Romance of Modern Invention Part 21

Photography has during the last forty years pa.s.sed through some startling developments, especially as regards speed. Experts, such as M. Marey, have proved the superiority of the camera over the human eye in its power to grasp the various phases of animal motion. Even rifle bullets have been arrested in their lightning flight by the sensitised plate. But while the camera is a valuable aid to the eye in the matter of form, the eye still has the advantage so far as colour is concerned. It is still impossible for a photographer by a simple process similar to that of making an ordinary black-and-white negative, to affect a plate in such a manner that from it prints may be made by a single operation showing objects in their natural colours. Nor, for the matter of that, does colour photography direct from nature seem any nearer attainment now than it was in the time of Daguerre.

There are, however, extant several methods of making colour photographs in an indirect or roundabout way. These various "dodges"

are, apart from their beautiful results, so extremely ingenious and interesting that we propose to here examine three of the best known.

The reader must be careful to banish from his mind those _coloured_ photographs so often to be seen in railway carriages and shop windows, which are purely the result of hand-work and mechanical printing, and therefore not _colour_ photographs at all.

Before embarking on an explanation of these three methods it will be necessary to examine briefly the nature of those phenomena on which all are based--light and colour. The two are really identical, light is colour and colour is light.

Scientists now agree that the sensation of light arises from the wave-like movements of that mysterious fluid, the omnipresent ether.

In a beam of white light several rates of wave vibrations exist side by side. Pa.s.s the beam through a prism and the various rapidities are sorted out into violet, indigo, blue, green, yellow, orange and red, which are called the pure colours, since if any of them be pa.s.sed again through a prism the result is still that colour. Crimson, brown, &c., the composite colours, would, if subjected to the prism, at once split up into their component pure colours.

There are several points to be noticed about the relationship of the seven pure colours. In the first place, though they are all allies in the task of making white light, there is hostility among them, each being jealous of the others, and only waiting a chance to show it.

Thus, suppose that we have on a strip of paper squares of the seven colours, and look at the strip through a piece of red gla.s.s we see only one square--the red--in its natural colour, since that square is in harmony only with red rays. (Compare the sympathy of a piano with a note struck on another instrument; if C is struck, say on a violin, the piano strings producing the corresponding note will sound, but the other strings will be silent.) The orange square suggests orange, but the green and blue and violet appear black. Red gla.s.s has arrested their ether vibrations and said "no way here." Green and violet would serve just the same trick on red or on each other. It is from this readiness to absorb or stop dissimilar rays that we have the different colours in a landscape flooded by a common white sunlight. The trees and gra.s.s absorb all but the green rays, which they reflect. The dandelions and b.u.t.tercups capture and hold fast all but the yellow rays. The poppies in the corn send us back red only, and the cornflowers only blue; but the daisy is more generous and gives up all the seven. Colour therefore is not a thing that can be touched, any more than sound, but merely the capacity to affect the retina of the eye with a certain number of ether vibrations per second, and it makes no difference whether light is reflected from a substance or refracted through a substance; a red brick and a piece of red gla.s.s have similar effects on the eye.

This then is the first thing to be clearly grasped, that whenever a colour has a chance to make prisoners of other colours it will do so.

The second point is rather more intricate, viz. that this imprisonment is going on even when friendly concord appears to be the order of the day. Let us endeavour to present this clearly to the reader. Of the pure colours, violet, green and red--the extremes and the centre--are sufficient to produce white, because each contains an element of its neighbours. Violet has a certain amount of indigo, green some yellow, red some orange; in fact every colour of the spectrum contains a greater or less degree of several of the others, but not enough to destroy its own ident.i.ty. Now, suppose that we have three lanterns projecting their rays on to the same portion of a white sheet, and that in front of the first is placed a violet gla.s.s, in front of the second a green gla.s.s, in front of the third a red gla.s.s. What is the result? A white light. Why? Because they meet _on equal terms_, and as no one of them is in a point of advantage no prisoners can be made and they must work in harmony. Next, turn down the violet lantern, and green and red produce a yellow, half-way between them; turn down red and turn up violet, indigo-blue results. All the way through a compromise is effected.

But supposing that the red and green gla.s.ses are put in front of the _same_ lantern and the white light sent through them--where has the yellow gone to? only a brownish-black light reaches the screen. The same thing happens with red and violet or green and violet.

Prisoners have been taken, because one colour has had to _demand pa.s.sage_ from the other. Red says to green, "You want your rays to pa.s.s through me, but they shall not." Green retorts, "Very well; but I myself have already cut off all but green rays, and if they don't pa.s.s you, nothing shall." And the consequence of the quarrel is practical darkness.

The same phenomenon may be ill.u.s.trated with blue and yellow. Lights of these two colours projected simultaneously on to a sheet yield white; but white light sent through blue and yellow gla.s.s _in succession_ produces a green light. Also, blue paint mixed with yellow gives green. In neither case is there darkness or entire cutting-off of colour, as in the case of Red + Violet or Green + Red.

The reason is easy to see.

Blue light is a compromise of violet and green; yellow of green and red. Hence the two coloured lights falling on the screen make a combination which can be expressed as an addition sum.

Blue = green + violet.

Yellow = green + red.

-------------------- green + violet + red = white.

But when light is pa.s.sed _through_ two coloured gla.s.ses in succession, or reflected from two layers of coloured paints, there are prisoners to be made.

Blue pa.s.ses green and violet only.

Yellow pa.s.ses green and red only.

So violet is captured by yellow, and red by blue, green being free to pa.s.s on its way.

There is, then, a great difference between the _mixing_ of colours, which evokes any tendency to antagonism, and the _adding_ of colours under such conditions that they meet on equal terms. The first process happens, as we have seen, when a ray of light is pa.s.sed through colours _in succession_; the second, when lights stream simultaneously on to an object. A white screen, being capable of reflecting any colour that falls on to it, will with equal readiness show green, red, violet, or a combination; but a substance that is in white light red, or green, or violet will capture any other colour. So that if for the white screen we subst.i.tuted a red one, violet or green falling simultaneously, would yield blackness, because red takes _both_ prisoners; if it were violet, green would be captured, and so on.

From this follows another phenomenon: that whereas projection of two or more lights may yield white, white cannot result from any mixture of pigments. A person with a whole boxful of paints could not get white were he to mix them in an infinitude of different ways; but with the aid of his lanterns and as many differently coloured gla.s.ses the feat is easy enough.

Any two colours which meet on equal terms to make white are called _complementary_ colours.

Thus yellow (= red + green lights) is complementary of violet.

Thus pink (= red + violet lights) is complementary of green.

Thus blue (= violet + green lights) is complementary of red.

This does not of course apply to mixture of paints, for complementary colours must act together, not in antagonism.

If the reader has mastered these preliminary considerations he will have no difficulty in following out the following processes.

(_a_) _The Joly Process_, invented by Professor Joly of Dublin. A gla.s.s plate is ruled across with fine parallel lines--350 to the inch, we believe. These lines are filled in alternately with violet, green, and red matter, every third being violet, green or red as the case may be. The colour-screen is placed in the camera in front of the sensitised plate. Upon an exposure being made, all light reflected from a red object (to select a colour) is allowed to pa.s.s through the red lines, but blocked by all the green and violet lines. So that on development that part of the negative corresponding to the position of the red object will be covered with dark lines separated by transparent belts of twice the breadth. From the negative a positive is printed, which of course shows transparent lines separated by opaque belts of twice their breadth. Now, suppose that we take the colour-screen and place it again in front of the plate in the position it occupied when the negative was taken, the red lines being opposite the transparent parts of the positive will be visible, but the green and violet being blocked by the black deposit behind them will not be noticeable. So that the object is represented by a number of red lines, which at a small distance appear to blend into a continuous whole.

The violet and green affect the plate in a corresponding manner; and composite colours will affect two sets of lines in varying degrees, the lights from the two sets blending in the eye. Thus yellow will obtain pa.s.sage from both green and red, and when the screen is held up against the positive, the light streaming through the green and red lines will blend into yellow in the same manner as they would make yellow if projected by lanterns on to a screen. The same applies to all the colours.

The advantage of the Joly process is that in it only one negative has to be made.

(_b_) _The Ives Process._--Mr. Frederic Eugene Ives, of Philadelphia, arrives at the same result as Professor Joly, but by an entirely different means. He takes three negatives of the same object, one through a violet-blue, another through a green, and a third through a red screen placed in front of the lens. The red negative is affected by red rays only; the green by green rays only, and the violet-blue by violet-blue rays only, in the proper gradations. That is to say, each negative will have opaque patches wherever the rays of a certain kind strike it; and the positive printed off will be by consequence transparent at the same places. By holding the positive made from the red-screen negative against a piece of red gla.s.s, we should see light only in those parts of the positive which were transparent. Similarly with the green and violet positives if viewed through gla.s.ses of proper colour. The most ingenious part of Mr. Ives' method is the apparatus for presenting all three positives (lighted through their coloured gla.s.ses) to the eye simultaneously. When properly adjusted, so that their various parts exactly coincide, the eye blends the three together, seeing green, red, or violet separately, or blended in correct proportions. The Kromoscope, as the viewing apparatus is termed, contains three mirrors, projecting the reflections from the positives in a single line. As the three slides are taken stereoscopically the result gives the impression of solidity as well as of colour, and is most realistic.

(_c_) _The Sanger Shepherd Process._--This is employed mostly for lantern transparencies. As in the Ives process, three negatives and three transparent positives are made. But instead of coloured gla.s.ses being used to give effect to the positives the positives themselves are dyed, and placed one on the top of another in close contact, so that the light from the lantern pa.s.ses through them in succession. We have therefore now quitted the realms of harmony for that of discord, in which prisoners are made; and Mr. Shepherd has had to so arrange matters that in every case the capture of prisoners does not interfere with the final result, but conduces to it.

In the first place, three negatives are secured through violet, green, and red screens. Positives are printed by the carbon process on thin celluloid films. The carbon film contains gelatine and bichromate of pota.s.sium. The light acts on the bichromate in such a way as to render the gelatine insoluble. The result is that, though in the positives there is at first no colour, patches of gelatine are left which will absorb dyes of various colours. The dyeing process requires a large amount of care and patience.

Now, it would be a mistake to suppose that each positive is dyed in the colour of the screen through which its negative was taken. A moment's consideration will show us why.

Let us a.s.sume that we are photographing a red object, a flower-pot for instance. The red negative represents the pot by a dark deposit. The positive printed off will consequently show clear gla.s.s at that spot, the unaffected gelatine being soluble. So that to dye the plate would be to make all red _except_ the very part which we require red; and on holding it up to the light the flower-pot would appear as a white transparent patch.

How then is the problem to be solved?

Mr. Shepherd's process is based upon an ordered system of prisoner-taking. Thus, as red in this particular case is wanted it will be attained by the _other two_ positives (which are placed in contact with the red positive, so that all three coincide exactly), robbing white light of all _but_ its red rays.

Now if the other positives were dyed green and violet, what would happen? They would not produce red, but by robbing white light between them of red, green, and violet, would produce blackness, and we should be as far as ever from our object.

The positives are therefore dyed, not in the same colours as the screens used when the negatives were made, but in their _complementary_ colours, _i.e._ as explained above, those colours which added to the colour of the screen would make white.

The red screen negative is therefore dyed (violet + green) = blue. The green negative (red + violet) = pink. The violet negative (red + green) = yellow.

To return to our flower-pot. The red-screen positive (dyed blue) is, as we saw, quite transparent where the pot should be. But behind the transparent gap are the pink and yellow positives.

White light (= violet + green + red) pa.s.ses through pink (= violet + red), and has to surrender all its green rays. The violet and red pa.s.s on and encounter yellow (= green + red), and violet falls a victim to green, leaving red unmolested.

If the flower-pot had been white all three positives would have contained clear patches unaffected by the three dyes, and the white light would have been un.o.bstructed. The gradations and mixtures of colours are obtained by two of the screens being influenced by the colour of the object. Thus, if it were crimson, both violet and red-screen negatives would be affected by the rays reflected by it, and the green screen negative not at all. Hence the pink positive would be pink, the yellow clear, and the blue clear.

White light pa.s.sing through is robbed by pink of green, leaving red + violet = crimson.

COLOUR PRINTING.

Printing in ink colours is done in a manner very similar to the Sanger Shepherd lantern slide process. Three blocks are made, by the help of photography, through violet, green and red screens, and etched away with acid, like ordinary half-tone black-and-white blocks. The three blocks have applied to them ink of a complementary colour to the screen they represent, just as in the Sanger Shepherd process the positives were dyed. The three inks are laid over one another on the paper by the blocks, the relieved parts of which (corresponding to the undissolved gelatine of the Shepherd positives) only take the ink.

White light being reflected through layers of coloured inks is treated in just the same way as it would be were it transmitted through coloured gla.s.ses, yielding all the colours in approximately correct gradations.