It will be noticed that the amount of black which it was necessary to introduce, in order to darken the true mixture of the colours so as to match the mixture of the pigments, was a very variable quantity, ranging from four to fifty-two per cent. It is for this reason that artists are so careful in their selection of pigments for the production of definite tones, particularly when they are to be luminous in quality. In four of these experiments it was found impossible to bring about equality without adding to the two original constituents a third colour, and in one case white had to be added; so that, in more than half the cases examined, the original colours were found incapable of reproducing by a true process of mixture the tint obtained on the palette without the aid of a foreign element. These experiments serve, then, to show that the results furnished by the palette can not be relied on to guide us in the interpretation or study of effects in nature depending on the mixture of coloured light.

We propose now to consider the results which are produced when a coloured surface is exposed to a coloured illumination and at the same time to white light. Effects of this kind are very common in nature, and are frequently purposely selected by artists as themes; in a minor degree they are always present to some extent, even when we seek to avoid them. With the knowledge which we have now gained, it is possible for us to recognize the fact that in such cases the resultant tint of the surface will depend on

three circumstances: first, on the colour which it assumes owing to the presence of the white light-that is to say, which it has owing to its natural or, as artists call it, "local colour"; secondly, on the colour communicated to it by that portion of the coloured light which is reflected unaltered from its surface; and to these there must be added, thirdly, the effects produced by the coloured light which penetrates below the surface, and is reflected after undergoing a certain amount of absorption. It is quite easy to make satisfactory experiments on this matter with the aid

FIG. 62.-White light from a window falls on stained glass at G, and is then concentrated by the lens on a sheet of white or coloured paper at P.

of a simple arrangement contrived by the author. At a distance of some eight or ten feet from a window, a lens, with a focal length of about five inches, is placed on a table, in such a way as to concentrate the white light from the window. In front of the lens a plate of coloured glass is held, and the result is that we obtain a bright beam of coloured light, which can be thrown on any coloured surface, such, for example, as painted paper (see Fig. 62). If

the walls of the room are white, the paper will at the same time be exposed to a white illumination; and, by turning it or removing it farther from the lens, the proportions of this double illumination can be varied at will. We will describe two experiments that were made with this arrangement: Yellow light was obtained by using a plate of glass which transmitted light having to the eye a pure yellow hue, without any tendency to orange-yellow or greenish-yellow. In this beam of light a piece of paper, painted with a very intense, deep hue of artificial ultramarine, was held. The portion illuminated by the yellow light appeared almost quite white, showing that a true mixture of the colours had taken place. It is well known that it is difficult to decide about the actual colour of a spot when it is surrounded by a coloured field; hence, in order to avoid deception by contrast, it is well in these experiments to observe the spot which has received the double illumination through an aperture cut in black paper, which is to be held in such a way as to permit a view only of this spot. This precaution was taken in the present case, and also in all the experiments that are given below. The ultramarine paper was then removed, and its place supplied by some which had been painted with Prussian-blue. The spot now appeared of a bright green colour, which proved that an action had taken place similar to that produced by mixing pigments on the palette. The explanation is as follows: The yellow glass transmits yellow, green, orange, and red light; and, as was explained in the previous chapter, these lights taken together make a light which appears to us yellow. That portion of this compound yellow light which penetrates the Prussian-blue undergoes a process of absorption; the green constituent, however, is not absorbed, and consequently is reflected rather abundantly from the paper. But some of the yellow light is reflected unaltered from the immediate surface of the paper; this mixes with the blue light (due to the white illumination), and makes white; so that what we

finally have is green mixed with more or less white. In the experiment where the ultramarine paper was used, no doubt some absorption took place, but it was not sufficient to modify the result materially; the blue and yellow light simply united, and formed white light. Below are given, in the form of tables, a large series of experiments made recently by the author; and an examination of them will show that for the most part the resultant tint depends rather on a true mixture of coloured lights, and that absorption acts only as a minor agent in modifying the results:

* Mixture of red lead and Indian-yellow.

+ Mixture of gamboge and Prussian-blue.

Mixture of emerald-green with a little chrome-yellow
§ Mixture of emerald-green with a little cobalt-blue.
Mixture of cobalt-blue and emerald-green.
Hoffmann's violet B. B.

** Hoffmann's violet B. B. and carmine.
++ Hoffmann's violet B. B. and carmine.
++ Lampblack.