MODERN OPTICS AND PAINTING. 



S77 



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 sim- 

 ple apparatus, with which you can easily repeat for yourselves many 

 of the experiments made to-night, as well as add greatly to their num- 

 ber. It consists merely of a plate of window-glass, of good quality, 

 set up on edge, and fastened on a blackened board (Fig. 7). If the 



Fig. 7. 



eye is placed at e, light will come to it directly from the blue square 

 of paper, JB\ but also at the same time light will reach it from the yel- 

 low 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 so- 

 lution 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 char- 

 acter 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 

 vol. iv. — 3*7 



