90
Light Waves and Their Uses
consists in counting the number of waves in a given distance. However, in counting such an enormous number, of the order of several hundred thousands, one is liable to make a blunder — not an error in a scientific sense, but a blunder. Of course, ultimately, this would be detected by the process of repetition.
The investigation, in a concrete form, presents a number of interesting ]>oints, involving problems of construction of a unique character which had to be solved before the process could be said to be perfectly successful.
The construction and operation of the apparatus will be much more readily understood if we first dwell a little upon the conditions that are to be fulfilled. Suppose, for illustration, that it is required to find the distance between two-mile posts on a railroad track. The most convenient method for measuring such a distance would be by a hundred-foot steel tape stretched by a known stretching force and applied to the steel rails. The rails are mentioned simply in order that there should not be any sag of the tape which would introduce still another error. The zero mark of the tape being placed against a mark on the rail which sorves as the starting-point, a second mark is made on the rail opposite the hundred-foot mark of the tape. The tape is then placed in position a second time with one end on the second mark, and a third mark is placed at the farther end; and so on indefinitely. This is the first process. By it we have divided the mile into the nearest whole number of hundred-foot spaces. Then we measure the fractions.
The second operation consists in verifying the length of the steel tape, which we must do by comparing it with a standard yard or foot by the same stepping-off process.
The process of measuring the meter in light waves is essentially the same as that described above, the meter answering to the distance of a mile of track, and the
Light Waves as Standards of Length 91
liundred-foot tape corresponding to a considerably smaller distance. This smaller distance is what I have termed an “ intermediate standard.” There is in this latter case the additional operation of finding the number of light waves in the intermediate standard; so that, in reality, there are three distinct processes to be considered.
In the first operation it is evident that, if an error is committed whenever we lift the tape and place it down again, the smaller the number of times wTe lift it and place it down, the smaller the total error produced; hence, one of the essential conditions of our apparatus would be to make this small standard as long as possible. The length of the intermediate standard is, however, limited by the distance at wThich wTe can observe interference fringes. The limit, as was stated in the last lecture, is reached when this distance is of the order of several hundred thousand waves. At this distance the interference fringes are rather faint, and it seemed better for such determinations not to make use of the extreme distance, but of such a smaller distance as would insure distinct interference fringes. It was found convenient to use, as the maximum length of the intermediate standard, one decimeter. The number of light waves in the difference of path (which is twTice the actual distance, because the light is reflected back) would be something of the order of three or four hundred thousand waves. With such a difference of path we can still see interference fringes comparatively clearly, if we choose the radiating substance properly.
The investigations described in the last lecture showed that the radiations emitted by quite a number of the substances which were examined were more or less highly complex. One remarkable exception, however, was found in the red radiation of cadmium vaj>or. This particular radiation proved to be almost ideally homogeneous, i, e., to con