Part 21 (1/2)

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 relations.h.i.+p 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.