22 THE INTERFEROMETRY OF 



8. Observations and experiments with a single grating. On considering 

 figure n, it will be seen that the doubly reflected, doubly diffracted rays are 

 also in a condition to interfere. Thus the rays GMGNG and GNGMG have 

 identical path-length, or at least path-difference; but it is improbable that 

 superimposed on the strong spectra this effect could be seen, for the reflec- 

 tion from the ruled face of the grating is very slight and the divergent 

 spectra have weakened seriously. The scintillating interferences, on the 

 other hand, are much brighter than the superposed spectra. Such interfer- 

 ences, also, should be independent of the play of the micrometer M , since 

 the path-difference of these beams is not changed thereby, each being identi- 

 cally lengthened or shortened. Furthermore, the interposition of a thick 

 plate-glass compensator in CM should have no effect. Neither of these infer- 

 ences applies for the phenomenon in question, which persists for a definite 

 displacement of M, only, and the introduction of a compensator requires the 

 usual equivalent displacement of M, within the range of the phenomenon. 

 Finally, the interferences relatively to a phenomenon produced by double 

 diffraction would not be modified. 



Many experiments were made to ascertain the path-difference within which 

 the phenomenon is visible. This can not be accurately determined, since it 

 is a question of stating when an observation, which is becoming rapidly less 

 distinct, has actually vanished. Moreover, any imperfection of the microm- 

 eter throws out the coincidence of longitudinal spectrum axes, while a read- 

 justment breaks the continuity of the micrometer displacement, or reading. 

 Results were obtained as follows, for example, AAT being the displacement of 

 the mirror M: 



With telescope AA 7 = 0.34, 0.45, 0.41 cm. 



With concave mirror and lens 0.45, 0.35, 0.41 cm. 



With concave mirror and adjustment 0.50 to 0.60 cm. 



The low readings are due to the micrometric wabbling of the micrometer 

 slide. Since A./V is the double path-difference, the number of wave-lengths 

 in question may be put 



6oXio- 6 



i.e., the distances along the ray are 15,000 to 20,000 wave-lengths apart, 

 about as estimated in the above paper. This is the characteristic feature of 

 the phenomenon. 



Between its extreme ranges of visibility the appearance of the phenomenon 

 scarcely changes. It ceases to be visible rather suddenly; and this is to be 

 expected, since we are dealing directly with two wave-trains displaced rela- 

 tively to each other. It is visible for a wide slit even after the Fraunhofer 

 lines vanish. It disappears by decreasing in width, when the slit is closed. 

 If the ocular of the telescope is drawn out, the phenomenon may even be 

 observed after the Fraunhofer lines have vanished, in the dark, stringy spec- 

 trum of an extremely fine slit. When the longitudinal axis of the spectrum 

 is indicated by a fine wire across the slit, the adjustment consists in bringing 



