Page:Popular Science Monthly Volume 1.djvu/192

182 seen: so the fault is doubled. I shall be able to show this more clearly by experiments.

The vertical and horizontal lines of the diagram (Fig. 1) are reflected with equal distinctness upon the screen by the spherical apparatus.

Those among my audience who have a decided form of astigmatism will, nevertheless, see them differently. Those whose sight is normal will only observe a difference after I have added a cylindrical lens to this apparatus, and thus made it astigmatical (Fig. 2). Ordinary spectacle-glasses are worked by a rotating movement on the surface of a sphere; cylindrical lenses are worked by moving the glass backward and forward upon a cylindrical surface. Such glasses produce an optical effect only in one direction. If instead of white lines I make the experiment with colored lines, it will show the mixing of colors produced by astigmatism; and if I now turn the axis of the lens, you will observe the effect of different forms of astigmatism. I show you a square (Fig. 3): if I add a cylindrical concave glass, with its axis placed horizontally, the square becomes an oblong.

In order now to show you how it is possible that the same eye may see an object at too great a distance elongated in a vertical direction, and, on the contrary, one that is too near enlarged in an horizontal direction, I need only place this cylindrical glass before or behind the focus of the apparatus without turning the axis, and you will then see the square, first elongated in a vertical direction (Fig. 4), and then enlarged in an horizontal direction.

Lastly, I show you a portrait. Imagine to yourself that it represents the person whom the astigmatical painter is painting; then, by aid of the cylindrical glass, you can form an idea how the painter sees this person.

If I alter the position of the glass, the portrait assumes the form in which the painter sees his own painting on the canvas. This will explain to you why he paints the portrait still longer than he sees the person.

With regard to an anomaly of sight, which seems almost foreign to the subject of painting—I mean color-blindness—I will also say a few words here, as the subject seems to be regarded with particular interest in England.

What we call color-blindness is a congenital defect of vision, which is characterized by the absence of one of the three primary sensations of color. The primary sensations of color are red, green, and violet, according to Thomas Young and Helmholtz; or red, green, and blue, according to Maxwell. When, as may easily happen, to this defect is joined a decided talent for painting, drawing alone ought to be attempted, because so absolute a defect will soon assert itself. But we meet with slighter degrees of color-blindness, where the perception of red is not entirely wanting, but only considerably diminished; so