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plate filter, on the assumption of the theory that every colour may 
be produced by suitable combinations of the three primary lights 
(not pigments), red, green, and violet. White light, as you know, 
is made up of a compound of seven colours—red, orange, yellow, 
green, blue, indigo, violet-—which is clearly demonstrated by 
dissecting a beam of sunlight through the spectrum, but it is 
known that it is not necessary to have all the seven colours to 
reform white light ; if we mix the red, green, and violet we shall get 
a very good white, which I will demonstrate to you by means of the 
triple lantern which we have here. In front of each lens is placed 
a piece of coloured glass, one red, one green, and one violet ; if 
they are focussed upon the screen the effect is white. I will also 
now show you the effect of mixing these colours. These three 
colours are known as the primary colours, and with these, by 
mixing in various proportions, we can make all the colours as 
seen in nature. I will focus the red and the green on the screen, 
and the effect as you will see is yellow. The red and violet when 
mixed produce pink. The green and the violet produce a green- 
blue. Now, if with any of these compound colours produced by 
the mixture of any two of the primary colours we add the other 
primary colour we get white, as you will see by focussing the 
violet and yellow—we get white (quite different to the mixing of 
pigments, which would give green). The pink and green give 
white. The green-blue and red also give white. This also 
explains what a complementary colour means, viz., supplying a 
deficiency, the particular deficiency being in the case of colour 
that which is necessary in order to reform white. 
As my time is limited I am sorry that I cannot explain to 
you how the sensation cf colour is produced to us, but colour is a 
physiological phenomenon produced by a physical difference in 
the wave length of light, a sensation formulated in the brain, 
depending entirely upon a difference in the wave length of light. 
Absorption of colour, too, is another phenomenon which would 
occupy too much of the time to go into this evening, but I may 
give you a short explanation of this by saying, that it is the pro- 
perty which a substance possesses of absorbing certain rays of 
light possessing a definite wave length. Grass is green because it 
possesses the faculty of absorbing all the rays of light except those 
which produce the sensation of green. 
Acting upon the idea of M. Ducos du Hauron, and the 
same theory, Messrs. Lumeire, of Lyons, have manufactured a 
photographic plate to record colour. A piece of glass is coated 
with a series of filters in the form of minute grains of starch dyed 
red, green, and violet. -Upon this is spread a film of emulsion 
which is sensitive to all parts of the visible spectrum. ‘The plate 
is placed in the dark slide with the glass side towards the lens, so 
that the light in order to reach it must pass through the coloured 
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