identity; that is to say, the image which is traced in our eye by real, objects is not identical with the image produced in our eye by the picture. This is best observed by changing the light. Whoever paints in London has but too frequent opportunities of observing this. A little more or less fog, the reflection of a cloud, illuminated by the sun, suffices to alter entirely the coloring of the picture, while the coloring of natural objects is not changed in the same manner.
Let us now return to our experiment with the yellow glass, and we shall find that it affects our eye very much in the same way as a yellow tint in the light, and therefore modifies natural objects in quite a different degree from pictures. If we continue the experiment for a considerable time, the difference becomes more and more essential. As I said before, the eye becomes dulled with regard to the yellow light, and thus sees Nature again in its normal coloring. The small quantity of blue light which is excluded by the yellow glass produces no sensible difference, as the difference is equalized by a diminution of sensibility with regard to yellow. In the picture, on the contrary, there is found in many places only as much blue as is perfectly absorbed by the yellow glass, and this, therefore, can never be perceived, however long we continue the experiment. Even for those parts of the picture which have been painted with the most intense blue the painter could produce, the quantity of blue excluded by the yellow glass will make itself felt, because its power is not so small with regard to pigments as with regard to the blue in Nature.
Imagine, now, that, in the course of years, one of the transparent media in the eye of a painter had gradually become yellowish, and that this yellow had by degrees considerably increased in intensity, and you will easily understand the influence it must exercise upon his work. He will see in Nature almost every thing correctly; but in his picture every thing will appear to him yellowish, and consequently he will paint it too blue. Does he not perceive this himself? Does he not believe it if told of it? Were this the case, it would be easy for him to correct the fault, since an artist can paint in a yellower or bluer tone, as he chooses. These are two questions which are easily answered by psychological experience. He does not perceive it himself, because he does not remember that he formerly saw in a different way. Our remembrance with regard to opinions, sensations, perceptions, etc., which have become gradually modified in the course of years—not by any external influence or sudden impression, but by a gradual change in our own physical or mental individuality—is almost nil.
He does not believe it—I would not say because an artist rarely recognizes what others tell him with regard to his works, but because with him, as with every one else, the impressions received through his own eye have a stronger power of conviction than any thing else. "Senen geht vor Sagen" ("Seeing is believing"), says the old adage.
