Page:Popular Science Monthly Volume 47.djvu/479

 This is shown in a modified way by the accompanying diagram.

If the page be tilted so as to be held in the line of vision, and if we "sight" along these heavy black lines, they are seen to be really parallel; and yet, when viewed as the page is ordinarily held, they appear inclined more or less to each other. Such an effect is produced only in exceptional cases, where the axis of the astigmatism varies in the two eyes, and this is therefore rather an unfair example. But the fact remains that the blurring of lines in certain directions may cause an artist to misrepresent very greatly the object which he is trying to reproduce.

Before leaving this phase of astigmatism it is worth while to note in passing a significant motion of the artist, already mentioned, in tipping his head from side to side, as he stands off to criticise his work. I am inclined to think he does this instinctively, in order to see better the errors in drawing caused by his own astigmatism.

The imperfect photographs of the Taj Mahal may serve to illustrate this point for some of the readers of this article. If one eye be closed (simply to exclude the correcting vision of the other), and if either of the astigmatic pictures be looked at while the book is rotated from side to side in the plane of the open page, one position can be found by most persons in which the lines are decidedly more distinct than in any other.

But, as the artist can not conveniently tip up the sides of his easel without disturbing also the equilibrium of brushes and paints and-bottles, he simply steps back and tips his head.

Moreover, the critic does the same. He, too, instinctively wishes to obtain the clearest lines—for to some these blurred, astigmatic images are confusing, disagreeable, almost painful—and to obviate that, the stranger in the studio, when he comes to see the finished picture, tips the head just as did the artist when he was at his work. The more closely we observe actions called "instinctive," the more frequently do we find they have an underlying cause.

There is another imperfection of vision, more frequently artificial and temporary than due to any structural change. This is