DISCOVERY 



151 



does not produce the same result as mixing or adding 

 together coloured pigments, of the same colours as the 

 lights. 



Coloured Lights. — Let us suppose we have a white 

 screen in a darkened room, and that by means of three 

 lanterns we are able to throw three spots of light upon 

 this screen. We can, for example, imagine we are in a 

 theatre or music-hall, and that the three lanterns are 

 situated in the front of the dress circle, each provided 

 with its own operator, that the screen is on the stage, 

 and that all lights are extinguished so that the theatre 

 is perfectly dark. It is, of course, not necessary for 

 the success of the experiment that it should be per- 

 formed on this ambitious scale, but the description is 

 probablv easier to understand when the e.xperiment is 

 performed in this way, instead of by one of the methods 

 used in the laboratory. We shall suppose that one of 

 the spots or discs of light on the otherwise dark screen 

 is coloured red, another green, and the third blue. 

 Then by tilting their lanterns the operators can make 

 these discs move across the screen and superimpose on 

 one another. Usually the coloured light in theatres 

 is produced by holding a sheet of coloured gelatine in 

 front of the lantern, but the coloured gelatine employed 

 in theatres is unsuitable for our experiment. The 

 colours are not nearly saturated enough, i.e. not intense 

 nor pure enough. Even the best green that the operator 

 throws upon the villain contains a good deal of white in 

 it. The red required for our experiment must be a pure 

 red without any tint of orange, somewhat similar to the 

 red of the railway signal lamps ; the green must contain 

 neither yellow nor blue, and must be pur.er than the 

 green of the signal lamps ; the blue must be an ultra- 

 marine with a good deal of violet in it. 



Under these circumstances, if the red is super- 

 imposed on the green we obtain yellow. Strong red 

 imposed on weak green gives orange, weak red on strong 

 green yellowish-green. Green superimposed on blue 

 gives peacock blue. Red superimposed on blue gives 

 magenta, and red on a stronger blue gives purple. 

 Red, green, and blue superimposed on one another 

 make white. If white is dimmed, we get grey ; if 

 orange is dimmed, we get brown. Superimposing white 

 on any colour makes it paler. Thus by means of the 

 three colours red, green, and blue we obtain nearly all 

 the colours that occur in nature. They don't give us 

 violet, but pure violets do not occur frequently in 

 nature. So red, green, and blue are termed the three 

 primary colours. Putting our results in the form of a 

 table we obtain : 



Red -f Green + Blue = White 

 Red -j- Green = Yellow \ 



Green -f Blue = Peacock Blue .- 

 Blue -!- Red = Magenta J 



Red + Peacock Blue = White | 

 Green -f- Magenta = White j- 

 Blue + Yellow = White j 



Peacock blue, magenta, and yellow are termed the 

 three complementaries, since each of them combined 

 with one of the primaries gives white. Peacock blue is 

 sometimes referred to as minus-red, since it is the colour 

 obtained by subtracting red from white, and in the 

 same way magenta and yellow are referred to as 

 minus-green and minus-blue. 



Coloured Piginenls. — We have now to consider the 

 mixture of coloured pigments. We are all to a certain 

 extent familiar with this subject owing to our experience 

 with water-colour paint-boxes when children ; we then 

 learned that yellow and blue make green, a result 

 which does not agree with those set forth in the table 

 above. \\'hence comes the contradiction ? 



A red object — for example, a stick of red sealing wax 

 — appears red because, when the three constituents of 

 white light — namely, red, green, and blue — fall on it 

 green and blue are absorbed, and red alone is reflected ; 

 in the same way a green object appears green, because 

 it absorbs the red and blue constituents of white light, 

 and only the green constituent is reflected. If a red 

 and a green paint are mixed, the mixture should 

 therefore absorb all three constituents of white light, 

 red, blue, and green, and consequently appear black, 

 instead of yellow, as we would obtain on mixing red 

 and green lights. A mixture of pigments gives only 

 the colour which neither absorbs, not the sum of the 

 two colours, as we obtain when adding lights. 



This can be illustrated very prettily by means of 

 the three complementaries, yellow, peacock blue, and 

 magenta. Stained gelatine film can be obtained from 

 Kodak, Ltd., Kingsway, London, E.G., exactly the 

 colour of the complementaries, and when these films are 

 combined two and two together, they give the colours 

 of the primaries with very great accuracy. It is 

 difficult to get paints or inks giving the colours of the 

 complementaries exactly right, but three of Dove's 

 waterproof inks, the yellow, the Prussian blue, and the 

 carmine, are fair approximations. Small bottles of 

 these inks can be obtained in most shops that supply 

 draughtsman's materials. They are brighter and 

 purer than the corresponding colours of the water- 

 colour paint-box. The effect of mixing these colours 

 can be studied best if they are added with a brush to 

 a little water in the bottom of an egg-cup. If the car- 

 mine, which absorbs only green, is added to the yellow, 

 which absorbs only blue, the mixture is red. If the 

 yellow, which absorbs only blue, is added to the Prussian 

 blue, the mixture is green. If a little carmine is added to 

 the Prussian blue, we get an ultramarine blue similar 

 to the third primary, and if we add more carmine we 



