REVERSED AND NON-REVERSED SPECTRA. 45 



18. Conclusion. To return, finally, to the original inference, it appears 

 that beating wave-trains have not been observed, but that the striking scin- 

 tillations are due to an exceptional susceptibility of the apparatus to labora- 

 tory tremors, when exhibiting the phenomenon in question. Of this I further 

 assured myself by observations made at night and on Sunday, though there is 

 some doubt in my mind. What has certainly been observed is the inter- 

 ference of a DI or DZ line with a reversed D'i or D' 2 line, both having the same 

 source and longitudinal axis. One can only assert, therefore, that light of 

 the wave-length interval of the breadth of these lines is capable of interference, 

 when the line is reversed. 



The phenomena, as a whole, are to be treated as diffractions of symmetrical 

 half wave-fronts, each of which may be separately controlled by the corre- 

 sponding micrometer. 



