that, for instance, an intense or strongly illuminated red can be perceived as such, while a less intense red appears green. This moderate degree of color-blindness does not always deter people from painting. A proof of this I saw at the last year's Exhibition, in a picture which represented a cattle-market. The roofs of the surrounding houses were all painted red on the sunny side, green in the shadow; but—what particularly struck me—the oxen also were red in the sun, green in the shadow. The slighter degrees of this anomaly, in the form of an insufficient perception of colors, have probably been the real cause why several great artists, who have become famous on account of the beauty of their drawing and the richness of their compositions, have failed to attain an equal degree of perfection in coloring.
In opposition to these isolated cases, I have to draw your attention to other cases which happen more frequently, and in advanced age, in consequence of a change in the perception of colors. They do not arise from a deficient function of the nervous apparatus of the eye, but in consequence of a change in the color of the lens.
The lens always gets rather yellow at an advanced age, and with many people the intensity of the discoloration is considerable. This, however, does not essentially diminish the power of vision. In order to get a distinct idea of the effect of this discoloration, it is best to make experiments with yellow glasses of the corresponding shade. Only, the experiment must be continued for some time, because at first every thing looks yellow to us. But the eye gets soon accustomed to the color, or rather it becomes dulled with regard to it, and then things appear again in their true light and color. This is at least the case with all objects of a somewhat bright and deep color. A careful examination, however, shows that a pale blue, or rather a certain small quantity of blue, cannot be perceived even after a very prolonged experiment, and after the eye has long got accustomed to the yellow color, because the yellow glass really excludes it. This must, of course, exercise a considerable influence when looking at pictures, on account of the great difference which necessarily exists between real objects and their representation in pictures.
These differences are many and great, as has been so thoroughly explained by Helmholtz. Let us for a moment waive the consideration of the difference produced by transmitting an object seen as a body on to a simple flat surface, and consider only the intensity of light and color. The intensity of light proceeding from the sun and reflected by objects is so infinitely greater than the strongest light reflected from a picture, that the proportion expressed in numbers is far beyond our comprehension. There is also so great a difference between the color of light, or of an illuminated object, and the pigments employed in painting, that it appears wonderful that the art of painting can, by the use of them, produce such perfect optical delusions. It can, of course, only produce optical delusions, never a real optical