Page:Popular Science Monthly Volume 47.djvu/476

462 is exclusively the result of a change in the eyes, which developed itself during the last twenty years of his life. In consequence of it, the aspect of Nature gradually changed for him, while he continued



in an unconscious, I might almost say in a naïve, manner to reproduce what he saw...."

That astigmatism distorts objects can be easily demonstrated. It is well known that the structure of the human eye is practically the same as that of the photographer's camera. Ordinarily the image which falls on the glass plate of the camera is equally clear in every part, because the lenses in front are ground with spherical surfaces. Such a camera when properly directed at a picture like that of the Taj Mahal, for example, gives us on the glass plate a clear and undistorted image of the building, such as is seen in Fig. 1. If, now, we render the front glass of the camera slightly astigmatic, by placing in front of it a so-called cylindrical glass with the axis horizontal, it produces optically exactly the same effect as that obtained when the globe of the eye is pressed from above downward. Moreover, the degree of this distortion in any eye can be reproduced with perfect exactness by placing in front of the camera a cylindrical glass of proper strength. It will be remembered that the average degree of astigmatism with the artist's examined was found to be