In the curve of Fig. 66 we have quite a contrast to the preceding. Here we have a radiation almost ideally homogeneous. Instead of having numerous maxima and minima like the curves we have been considering, this visibility curve diminishes very gradually according to a very simple mathematical law, which tells us that the source of light is a single line of extremely small breadth, the breadth being of the order of one eight-hundredth to one-thousandth of theFIG. 66 distance between the sodium lines. It is impossible to indicate exactly the width of the line, because the distribution of intensity throughout it is not uniform. The important point to which I wish to call attention, however, is that this curve is of such a simple character that for a difference of path of over 200 millimeters, or 400,000 light waves, we can obtain interference fringes. This indicates that the waves from this source are almost perfectly homogeneous. It is therefore possible to use these light waves as a standard of length, as will be shown in a subsequent lecture. The curve corresponds to the red radiation from cadmium vapor in a vacuum tube. In using this red cadmium wave as a standard of length it is very important to have other radiations by which we can check our observations. The cadmium has two other lines, which serve as a control or check to the result obtained by the first.

Fig. 67 represents the green radiation of cadmium. This curve is not quite so simple as that of the red, but