We are almost always conscious of indistinct vision, be it in consequence of incorrect accommodation or insufficient power of sight, especially if it is not congenital, but has gradually appeared. But it is extremely difficult and in many cases impossible to convince those of their defect who suffer from incorrect vision as to form and color. They never become conscious of it themselves, even if it is not congenital, and the most enlightened and intelligent among them remain incredulous, or become even angry and offended, when told of it. Incorrect perception of form may, however, easily be demonstrated. If in consequence of astigmatism a square appears oblong to any one, he can measure the sides with a compass; or, what is more simple still, he can turn it so that the horizontal lines are changed into vertical ones, and vice versa, and his own sight will convince him of his error. It is more difficult to demonstrate whether a person sees colors correctly or not. Such glaring mistakes as those produced by color-blindness can be easily recognized, but faults produced by a diminished sensation of small differences in the shades of color can only be recognized as such by the fact that the majority of persons with normal vision declare them to be faults. Such, for instance, are deviations produced by an incorrect perception of pigments, which in painting makes itself felt by a constantly-recurring plus or minus of a single color in the whole picture. It may also show itself by small faults in the rendering of every color. In discussing this subject with artists, they at once declare these anomalies to represent a school, a taste, a manner, which may be arbitrarily changed. They most unwillingly concede that peculiarities of sight have any thing to do with it. It seems to me sometimes as if they considered it in a certain measure a degradation of their art that it should be influenced by an organ of sense, and not depend entirely upon free choice, intelligence, imagination, and talent.
Thus, to return to the point from which we started: if a painter whose lens becomes yellower begins to paint in a bluer tone, it is said that he has changed his style. The painter himself vehemently protests against this opinion; he thinks that he still paints in his old style, and that he has only improved the tone of his color. His earlier works appear to him too brown. To convince him of his error, it would be necessary to remove his lens suddenly. Then every thing would appear to him too blue, and his paintings far too blue. This is no hypothesis, but a fact. Patients on whom I have operated for cataract, very often spontaneously declared, immediately after the operation, that they saw every thing blue; in these cases I invariably found their crystalline lens to be of an intense yellow color. In pictures painted after the artists were considerably over sixty, the effect of the yellow lens can often be studied. To me their pictures have so characteristic a tone of color, that I could easily point them out while passing through a picture-gallery. As a striking example,
