Part 20

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 passed 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 substituted 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 glasses 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 glass 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 pass 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 passage 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 glass, we should see light only in those parts of the positive which were transparent. Similarly with the green and violet positives if viewed through glasses of proper colour. The most ingenious part of Mr. Ives’ method is the apparatus for presenting all three positives (lighted through their coloured glasses) 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 glasses 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 passes 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 potassium. 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 assume 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 glass 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) passes through pink (= violet + red), and has to surrender all its green rays. The violet and red pass 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 unobstructed. 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 passing 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 glasses, yielding all the colours in approximately correct gradations.

LIGHTING.

The production of fire by artificial means has been reasonably regarded as the greatest invention in the history of the human race. Prior to the day when a man was first able to call heat from the substances about him the condition of our ancestors must have been wretched indeed. Raw food was their portion; metals mingled with other matter mocked their efforts to separate them; the cold of winter drove them to the recesses of gloomy caverns, where night reigned perpetual.

The production of fire also, of course, entailed the creation of light, which in its developments has been of an importance second only to the improved methods of heating. So accustomed are we to our candles, our lamps, our gas-jets, our electric lights, that it is hard for us to imagine what an immense effect their sudden and complete removal would have on our existence. At times, when floods, explosions, or other accidents cause a temporary stoppage of the gas or current supply, a town may for a time be plunged into darkness; but this only for a short period, the distress of which can be alleviated by recourse to paraffin lamps, or the more homely candle.

The earliest method of illumination was the rough-and-ready one of kindling a pile of brushwood or logs. The light produced was very uncertain and feeble, but possibly sufficient for the needs of the cave-dweller. With the advance of civilisation arose an increasing necessity for a more steady illuminant, discovered in vegetable oils, burned in lamps of various designs. Lamps have been found in old Egyptian and Etruscan tombs constructed thousands of years ago. These lamps do not differ essentially from those in use to-day, being reservoirs fitted with a channel to carry a wick.

But probably from the difficulty of procuring oil, lamps fell into comparative disuse, or rather were almost unknown, in many countries of Europe as late as the fifteenth century; when the cottage and baronial hall were alike lit by the blazing torch fixed into an iron sconce or bracket on the wall.

The rushlight, consisting of a peeled rush, coated by repeated dipping into a vessel of melted fat, made a feeble effort to dispel the gloom of long winter evenings. This was succeeded by the tallow and more scientifically made wax candle, which last still maintains a certain popularity.

How our grandmothers managed to “keep their eyes” as they worked at stitching by the light of a couple of candles, whose advent was the event of the evening, is now a mystery. To-day we feel aggrieved if our lamps are not of many candle-power, and protest that our sight will be ruined by what one hundred and fifty years ago would have seemed a marvel of illumination. In the case of lighting necessity has been the mother of invention. The tendency of modern life is to turn night into day. We go to bed late and we get up late; this is perhaps foolish, but still we do it. And, what is more, we make increasing use of places, such as basements, underground tunnels, and “tubes,” to which the light of heaven cannot penetrate during any of the daily twenty-four hours.

The nineteenth century saw a wonderful advance in the science of illumination. As early as 1804 the famous scientist, Sir Humphrey Davy, discovered the electric arc, presently to be put to such universal use. About the same time gas was first manufactured and led about in pipes. But before electricity for lighting purposes had been rendered sufficiently cheap the discovery of the huge oil deposits in Pennsylvania flooded the world with an inexpensive illuminant. As early as the thirteenth century Marco Polo, the explorer, wrote of a natural petroleum spring at Baku, on the Caspian Sea: “There is a fountain of great abundance, inasmuch as a thousand shiploads might be taken from it at one time. This oil is not good to use with food, but it is _good to burn_; and is also used to anoint camels that have the mange. People come from vast distances to fetch it, for in all other countries there is no oil.” His last words have been confuted by the American oil-fields, yielding many thousands of barrels a day--often in such quantities that the oil runs to waste for lack of a buyer.

The rivals for pre-eminence in lighting to-day are electricity, coal gas, petroleum, and acetylene gas. The two former have the advantage of being easily turned on at will, like water; the third is more generally available.

The invention of the dynamo by Gramme in 1870 marks the beginning of an epoch in the history of illumination. With its aid current of such intensity as to constantly bridge an air-gap between carbon points could be generated for a fraction of the cost entailed by other previous methods. Paul Jablochkoff devised in 1876 his “electric candle”--a couple of parallel carbon rods separated by an insulating medium that wasted away under the influence of heat at the same rate as the rods. The “candles” were used with rapidly-alternating currents, as the positive “pole” wasted twice as quickly as the negative. During the Paris Exhibition of 1878 visitors to Paris were delighted by the new method of illumination installed in some of the principal streets and theatres.

The arc-lamp of to-day, such as we see in our streets, factories, and railway stations, is a modification of M. Jablochkoff’s principle. Carbon rods are used, but they are pointed towards each other, the distance between their extremities being kept constant by ingenious mechanical contrivances. Arc-lamps of all types labour under the disadvantage of being, by necessity, very powerful; and were they only available the employment of electric lighting would be greatly restricted. As it is, we have, thanks to the genius of Mr. Edison, a means of utilising current in but small quantities to yield a gentler light. The glow-lamp, as it is called, is so familiar to us that we ought to know something of its antecedents.

In the arc-lamp the electric circuit is _broken_ at the point where light is required. In glow or incandescent lamps the current is only _hindered_ by the interposition of a bad conductor of electricity, which must also be incombustible. Just as a current of water flows in less volume as the bore of a pipe is reduced, and requires that greater pressure shall be exerted to force a constant amount through the pipe, so is an electric current _choked_ by its conductor being reduced in size or altered in nature. Edison in 1878 employed as the current-choker a very fine platinum wire, which, having a melting temperature of 3450 degrees Fahrenheit, allowed a very white heat to be generated in it. The wire was enclosed in a glass bulb almost entirely exhausted of air by a mercury-pump before being sealed. But it was found that even platinum could not always withstand the heating effect of a strong current; and accordingly Edison looked about for some less combustible material. Mr. J. W. Swan of Newcastle-on-Tyne had already experimented with carbon filaments made from cotton threads steeped in sulphuric acid. Edison and Swan joined hands to produce the present well-known lamp, “The Ediswan,” the filament of which is a bamboo fibre, carbonised during the exhaustion of air in the bulb to one-millionth of an atmosphere pressure by passing the electric current through it. These bamboo filaments are very elastic and capable of standing almost any heat.

Glow-lamps are made in all sizes--from tiny globes small enough to top a tie-pin to powerful lamps of 1000 candle-power. Their independence of atmospheric air renders them most convenient in places where other forms of illumination would be dangerous or impossible; _e.g._ in coal mines, and under water during diving operations. By their aid great improvements have been effected in the lighting of theatres, which require a quick switching on and off of light. They have also been used in connection with minute cameras to explore the recesses of the human body. In libraries they illuminate without injuring the books. In living rooms they do not foul the air or blacken the ceiling like oil or gas burners. The advantages of the “Edison lamp” are, in short, multitudinous.

Cheapness of current to work them is, of course, a very important condition of their economy. In some small country villages the cottages are lit by electricity even in England, but these are generally within easy reach of water power. Mountainous districts, such as Norway and Switzerland, with their rushing streams and high water-falls, are peculiarly suited for electric lighting: the cost of which is mainly represented by the expense of the generating apparatus and the motive power.

One of the greatest engineering undertakings in the world is connected with the manufacture of electric current. Niagara, the “Thunder of the Waters” as the Indians called it, has been harnessed to produce electrical energy, convertible at will into motion, heat, or light. The falls pass all the water overflowing from nearly 100,000 square miles of lakes, which in turn drain a far larger area of territory. Upwards of 10,000 cubic yards of water leap over the falls every second, and are hurled downwards for more than 200 feet, with an energy of eight or nine million horse-power! In 1886 a company determined to turn some of this huge force to account. They bought up land on the American bank, and cut a tunnel 6700 yards long, beginning a mile and a half above the falls, and terminating below them. Water drawn from the river thunders into the tunnel through a number of wheel pits, at the bottom of each of which is a water-turbine developing 5000 horse-power. The united force of the turbines is said to approximate 100,000 horse-power; and as if this were but a small thing, the same Company has obtained concessions to erect plant on the Canadian bank to double or treble the total power.

So cheaply is current thus produced that the Company is in a position to supply it at rates which appear small compared with those that prevail in this country. A farthing will there purchase what would here cost from ninepence to a shilling. Under such conditions the electric lamp need fear no competitor.

But in less favoured districts gas and petroleum are again holding up their heads.

Both coal and oil-gas develop a great amount of heat in proportion to the light they yield. The hydrogen they contain in large quantities burns, when pure, with an almost invisible flame, but more hotly than any other known gas. The particles of carbon also present in the flame are heated to whiteness by the hydrogen, but they are not sufficient in number to convert more than a fraction of the heat into light.

A German, Auer von Welsbach, conceived the idea of suspending round the flame a circular “mantle” of woven cotton steeped in a solution of certain rare earths (_e.g._ lanthanum, yttrium, zirconium), to arrest the heat and compel it to produce bright incandescence in the arresting substance.

With the same gas consumption a Welsbach burner yields seven or more times the light of an ordinary batswing burner. The light itself is also of a more pleasant description, being well supplied with the blue rays of the spectrum.

The mantle is used with other systems than the ordinary gas-jet. Recently two methods of illumination have been introduced in which the source of illumination is supplied under pressure.

The high-pressure incandescent gas installations of Mr. William Sugg supply gas to burners at five or six times the ordinary pressure of the mains. The effect is to pulverise the gas as it issues from the nozzle of the burners, and, by rendering it more inflammable, to increase its heating power until the surrounding mantle glows with a very brilliant and white light of great penetration. Gas is forced through the pipes connected with the lamps by hydraulic rams working gas-pumps, which alternately suck in and expel the gas under a pressure of twelve inches (_i.e._ a pressure sufficient to maintain a column of water twelve inches high). The gas under this pressure passes into a cylinder of a capacity considerably greater than the capacity of the pumps. This cylinder neutralises the shock of the rams, when the stroke changes from up-to downstroke, and _vice versâ_. On the top of the cylinder is fixed a governor consisting of a strong leathern gas-holder, which has a stroke of about three inches, and actuates a lever which opens and closes the valve through which the supply of water to the rams flows, and reduces the flow of the water when it exceeds ten or twelve inches pressure, according to circumstances. The gas-holder of the governor is lifted by the pressure of the gas in the cylinder, which passes through a small opening from the cylinder to the governor so as not to cause any sudden rise or fall of the gas-holder. By this means a nearly constant pressure is maintained; and from the outlet of the cylinder the gas passes to another governor sufficient to supply the number of lights the apparatus is designed for, and to maintain the pressure without variation whether all or a few lamps are in action. For very large installations steam is used.

Each burner develops 300 candle-power. A double-cylinder steam-engine working a double pump supplies 300 of these burners, giving a total lighting-power of 90,000 candles. As compared with the cost of low-pressure incandescent lighting the high-pressure system is very economical, being but half as expensive for the same amount of light.

It is largely used in factories and railway stations. It may be seen on the Tower Bridge, Blackfriars Bridge, Euston Station, and in the terminus of the Great Central Railway, St. John’s Wood.

Perhaps the most formidable rival to the electric arc-lamp for the lighting of large spaces and buildings is the Kitson Oil Lamp, now so largely used in America and this country.