25 × 100 75 I'

I' = 33.3

RED

GREEN-BLUE

FIG. 74.-Disk with 25 Parts Red and 75 Parts Green-blue.

That is, if we call the intensity of our red 100, that of the greenblue will be only 33.3. In the case given in the present chapter we have

41 I = 59 I'

41 x 100 59 I'

I' = 69.5

PIGMENTS USED IN THE SET OF COMPLEMENTARY DISKS.

Carmine as a water-colour; for its complementary green-blue, a mixture of cobalt-blue and emerald-green.

Vermilion as a water-colour; for its complement the same as above, the proportions being changed.

For the first two pairs, then, we can employ two of our most intense and saturated pigments; this, however, is not possible with orange and yellow, without producing disks of a rank different from the preceding, or obtaining disks which show greater differences in luminosity than any which have been tolerated in the table given in the present chapter. Thus a fine orange colour was mixed from red lead and Indian-yellow, which would have been considered by most painters, as I suppose, a fair companion for the carmine and vermilion; or, if objection had been made, it would have been rather to its want of intensity. Placing the intensity of this orange as 100, the intensity of its complement (made of cobalt-blue and emerald-green) was only 47, a figure smaller than any in the table.

These two colours, however, furnished a white such as could be obtained by mixing, with the aid of disks, 36 parts of white with 64 of black; this number is considerably higher than those allowed in the table. The combination then was rejected, because it was faulty in two respects, and a dull-looking orange substituted for it. This dull, rather poor-looking orange balanced its complementary cyanblue well, and with it gave 27 per cent. of white light, which was fully up to the average, and proved that in the matter of luminosity it belonged in the set rather than the disk just mentioned.

A similar experience was encountered with yellow. Two beautiful disks were prepared with gamboge and cobalt-blue. Setting the intensity of the gamboge as 100, that of the cobalt was 90, which was nearly what was wanted. The combination, however, gave on rotation a white which was about 100 per cent. too bright, showing that the two disks belonged in a set such as would be furnished by pigments twice as bright as those employed by me; but no such pigments exist. This is only another illustration of the fact, already several times mentioned, that our bright-yellow pigments, such as gamboge, chrome-yellow, cadmium-yellow, etc., can not properly be reckoned as the equal companions of the other pigments ordinarily found on the painter's palette. This circumstance affects our judgment, and we are surprised at the lack of brilliancy of the yellow space even in the prismatic spectrum, and at the fact that mixtures of red and green light produce yellow light of so inferior a character. On the other hand, the possession of such exceptional pigments as the bright yellows and orange-yellows enables the artist at will to extend his scale of brilliancy in an upward direction much farther than otherwise would be possible.

The greenish-yellows were made with gamboge mixed with a little Prussian-blue, the pigments being laid, not on drawing-paper, but on rather absorbent cardboard, which dulled the colours to a desirable extent. For violet, "Hoffmann's violet B. B." was employed, none of the violet pigments used by artists being of the slightest use on account of their very dull appearance and poverty in the matter of violet light. The green was made by mixing a little chrome-yellow with emerald-green; the purple was "Hoffmann's violet R. R. R."

CHAPTER XII.

ON THE EFFECT PRODUCED ON COLOUR BY A CHANGE IN LUMINOSITY, AND BY MIXING IT WITH WHITE LIGHT.

In our study thus far of coloured surfaces it has been tacitly assumed that their action on the eye is a constant one, and that a red surface, for example, will always appear red to a healthy eye as long as it remains visible. In point of fact, however, this is not quite true, for it is found that coloured surfaces undergo changes of tint when they are seen under a very bright or very feeble illumination. Artists are well aware that scarlet cloth under bright sunshine approaches orange in its tint; that green becomes more yellowish; and that, in general, a bright illumination causes all colours to tend somewhat toward yellow in their hues. Helmholtz, Bezold, Rutherfurd, and others have made similar observations on the pure colours of the prismatic spectrum, and have found that even they undergo changes analogous to those just indicated. The violet of the spectrum is very easily affected: when it is feeble (that is, dark), it approaches purple in its hue; as it is made stronger, the colour changes to blue, and finally to a whitish-grey with a faint tint of violet-blue. The changes with the ultramarine-blue of the spectrum follow the same order, passing first into sky-blue, then into whitish-blue, and finally into white. Green as it is made brighter passes into yellowish-green, and then into whitish-yellow; for actual conversion into white it is necessary that the illumination should

be dazzling. Red resists these changes more than the other colours; but, if it be made quite bright, it passes into orange and then into bright yellow.

It is remarkable that these changes take place with the pure colours of the spectrum; but the explanation, according to the theory of Young and Helmholtz, is not difficult. Let us illustrate it by an example, taking the case of green light, which, as we have seen, acts most powerfully on what we termed the green nerves, less powerfully on the red and violet nerves. Now, as long as the intensity of our green light is small, it acts almost entirely on its own peculiar set of nerves; but, when the green light is made brighter, it begins to set into action also the red and to a lesser extent the violet nerves; the result of this is that the sensation of white begins to be mingled with that of green, all three sets of nerves being now to some extent in action. As in this process the violet nerves lag behind, the main modification of the colour at this stage is due to the action of the red nerves, which cause it to appear more yellowish ; hence it changes first to a yellowish-green, then to greenishyellow, and finally, if the light is very bright, to a whitishyellow. Corresponding to this, when red light is made very bright, the red and the green nerves are set into action, the result being that the colour changes in appearance from red to yellow. In this case the violet nerves play a secondary part, and their action merely causes this yellow to appear somewhat whitish. When pure violet light is made quite bright, immediately the green nerves begin to add their action to that of the violet, and the tint quickly changes from violet to ultramarine-blue; the red nerves are soon also stimulated, and, in connection with the green, furnish the sensation of yellow; this yellow, mixing with that of the ultramarine-blue before mentioned, gives as a resultant tint a whitish-grey with a faint tint of blue or violetblue. The explanation of the changes which the intermediate colours of the spectrum undergo is analogous to that

just given. The tendency in all cases is to the production of a yellowish-white, or to a white, if the coloured light be very bright. If its brightness be more moderate, the colour will still appear paler and as though mixed with a certain amount of yellow. Artists, by taking advantage of these facts, are able to represent in their paintings scenes under high degrees of illumination. According to Aubert, the whitest white paper is only 57 times brighter than the darkest black paper; and it is within these narrow limits that the painter is compelled to execute his design: hence the necessity of employing illusions like the one just mentioned. Many effects in nature are beautiful and striking, as much on account of their high degree of luminosity as for any other reason. The artist is not able to transfer to his canvas the brightness, which in this case is really the attractive element; but by the use of pale colours, well modulated, he suggests a flood of light, and we are delighted, not so much with the pale tints as with the recollections they call up.

We have just examined the remarkable alterations which the pure colours of the spectrum undergo when their luminosity is made very great, and pass now to the changes which occur when the intensity of coloured light is made very feeble. Von Bezold has made some interesting observations of this character on the colours of the spectrum. With a very bright prismatic spectrum he was able to see a pure yellow near D and a whitish-blue near F, the other colours being in their usual positions. When the illumination was only moderately bright, the yellow space diminished and became very narrow; the ultramarine-blue vanished, and was replaced by violet. With less illumination, the orange-yellow space assumed the colour of red lead, and the yellow vanished, being replaced by a greenish tint; the cyan-blue was replaced by green, the blue and ultramarineblue by violet. The spectrum at this stage presented scarcely more than the three colours, red, green, and violet. With