tions are horizontal, while in the outer three on both sides the vibrations are vertical. An idea of the power of this instrument can be obtained by comparing Fig. 81 with Fig. 80, which gives the appearance of the line as seen in the best grating spectroscope.
SUMMARY
1. The investigation of the changes produced in the radiations of substances by placing them in the magnetic field is in general a phenomenon barely within the range of the best spectroscopes, and there are some features of it which it would be entirely hopeless to attack by this method.
2. Such investigations, however, are precisely the kind for which the interference method is particularly adapted. In fact, the results of the investigation by the method of visibility curves have furnished a number of new and interesting developments which could only with difficulty have been obtained by the ordinary spectrometer methods.
3. Fertile as this method has shown itself to be, there are, nevertheless, a number of serious drawbacks. In order to obviate these a new instrument was devised, the echelon spectroscope, which has all the advantages of the grating spectroscope, together with a resolving power many times as great. With the aid of this instrument all the preceding deductions have been amply verified and a number of new and interesting facts added to the store of our knowledge of the Zeeman effect.
