According to our situation, our shadow is long or short, narrow or broad, it falls to the right or to the left, and all these things have a significance for the work upon which we are engaged. If we can compare the shadows of different people, we shall see that they differ from each other. Some are darker, some are lighter. It is very difficult to note the degree of depth of shadow, just as it is difficult to distinguish which of several shades of a dark stuff is the darkest. But if there were some machine which could register the depth of the shadows, we should be aware that they differ.