when exposed to a double illumination, or when illuminated by light having a hue different from that of the surface itself. Applications of it will be given at a later stage.
Before leaving this part of the subject, I wish to show a very simple apparatus, with which you can easily repeat for yourselves many of the experiments made tonight, as well as add greatly to their number. It consists merely of a plate of window-glass, of good quality, set up on edge, and fastened on a blackened board (Fig. 1). If the
Fig. 7.
eye is placed at e, light will come to it directly from the blue square of paper, B, but also at the same time light will reach it from the yellow square of paper, Y; and these two masses of colored light, being mingled on the retina of the eye, will produce the same effects which I have just exhibited to you with much more costly apparatus. You will also find that you can vary the brightness of either of your squares by adjusting them at a greater or less distance from the plate of glass. When they are near to it, the yellow will predominate; the blue, when they are farther from it. Great use was made by Helmholtz of this contrivance in his experiments on this subject, and you will easily be able to prove for yourselves that the red light from paper painted with vermilion, when combined with the green light from the water-colored pigment known as "emerald-green," gives a yellowish or orange tint, according as the apparatus is arranged. Chrome-yellow (the pale variety) and ultramarine-blue give an excellent white. It is somewhat difficult to obtain a good representative of violet from among the colors in use by artists. I find that some samples of the dyeing material known as "Hoffmann's violet BB" answer better than any of the ordinary pigments. If a deep tint of its alcoholic solution be spread over paper, and combined in the instrument with emerald-green, a blue, greenish-blue, or violet-blue, can be readily produced. It is evident that a multitude of experiments of this character can be made, the number of colors united at one time being limited to two. For certain purposes I have modified the apparatus so that three tints can be combined. A second plate of glass is added at P, Fig. 8; this allows the compound beam of light from the first
