18 REVERSED AND NON-REVERSED SPECTRA. 



Hence, since X 2 is very small and x relatively very large, the small value of 

 AX (i.e., the very thin strip of spectrum within which the phenomenon occurs) 

 is apparent. In the above experiments the estimates, in round numbers, 

 were AX = 2.4Xicr 8 , X 2 = 36Xio~ 10 . Hence if n = i, 



# = 36Xicr 10 /2.4Xicr 8 = .i5 cm. 



so that one reenforcement would have to occur about at each 1.5 mm. along 

 the rays. Nevertheless, the formidable difficulty remains to be investigated, 

 viz, why these nominally beating wave-trains, with an infinitesimal group 

 period (icr 11 sec.), could be recognized at all. 



The characteristic feature of the new phenomenon is this, that apart from 

 intensity it persists, without variation, through a path-difference of over 5 milli- 

 meters; i.e., through 15,000 or 20,000 wave-lengths. It follows, since the 

 optical paths grating-mirror-grating are alone significant, that two individual 

 light -waves of the same ray over 15,000 wave-lengths apart are still appre- 

 ciably identical. Beyond that the waves under consideration no longer 

 correspond in orientation and can not interfere in a way to produce alterna- 

 tions of accentuated brightness and darkness. 



