glass would be one and one-half times as great, so that the difference in path would be 25,000 waves. But the resolving power is the order of spectrum multiplied by the number of plates. If we are observing, therefore, in the 25,000th spectrum, and there are thirty such plates, the resolving power would be 750,000; whereas the resolving power of the best gratings is about 100,000.
There are, however, disadvantages in the use of this instrument. One of these may be illustrated as follows: Suppose we take the case of the ordinary grating; the first spectral image is rather widely separated from the central image of the slit, the second spectral image is twice as far away as the first, and the third spectral image will start three times as far away as the first, and will also be three times as long. The result is that parts of the second and third overlap. The overlapping becomes greater and greater as the order of the spectrum increases, so that when the 25,000th spectrum is reached the spectra are inextricably confused. Where we have to deal with a few simple radiations, however, as in cadmium or sodium, this overlapping is not so serious as might be supposed. We have a very simple means of getting rid of the worst of it by analyzing the light by means of a prism before it enters the pile of plates.
The construction of the instrument is not very different from that of the ordinary spectroscope. The light passes through a slit and then through a lens, by which it is made parallel. It then passes through the pile of plates—the echelon, as it has been named—and into the observing telescope. With this instrument the results obtained by the method of visibility curves have been confirmed. Thus Fig. 81 shows the appearance of the green mercury line in the field of view of the echelon when the source is in a strong magnetic field. In the three central components the vibra-
