Well, if you have carefully studied your portrait in the mirror, I think you must have discovered how large a part the contrast of light and shadow plays in the appearance of the figure, and therefore, what an equally important part it plays in producing an illusion of reality in the picture. I do not forget that an artist by simply drawing an outline with a pen or pencil can also suggest to us the appearance of an object. But, if he does so, it is by the help of ourselves, for he relies on our imagination to supply what he has omitted.