498 0. Barus — Interferences of Crossed Spectra. 



In this place, however, it is my sole purpose to present, at 

 its face value, an observation which is spacial, independent of 

 time consideration, and the laterally cramped character of the 

 new interference, with its long hair-like lines thrust into a strip 

 less than half the distance apart of the sodium lines, is the only 

 evidence submitted. For, if the coincident path of two rays 

 of slightly different wave-lengths X and X' which interfere, 

 is x, then there are x/X and x/\' complete waves in the given 

 path ; and in case of original identity in phase, reinforcement 

 will occur when 



sb(1/X— 1/A') = 1, 2, 3, ...n. 

 In other words at the nth reinforcement 



AA = nX'/x. 



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

 exceedingly small value AX (i. e. the very narrow strip of 

 spectrum within which the phenomenon occurs) is apparent. 

 In the above experiments the estimates, in round numbers, were 

 AX = 2-4 X 10- 8 , X 2 = 36 X lO"' . Hence if n = l,x=-15 cm. 

 so that one reinforcement occurs about at each 1*5 millimeter 

 along the rays. 



It remains to be investigated why these nominally beating 

 wave trains with an infinitesimal group period, can be recog- 

 nized at all. But into this question I am not now prepared to 

 enter, as the answer is almost wholly dependent on experi- 

 ments in progress. If whatever vibrates has inertia (electric 

 displacement), the case of forced vibrations is suggested 

 (approximate resonance), with opposite phases on the two sides 

 of the transverse line of coincidence, essential. 



The characteristic feature of the new phenomenon is this, 

 that apart from intensity, it persists without variation, through 

 a path difference of over 10 millimeters, i. e. through 15,000 or 

 20,000 wave lengths. It follows, since the paths, grating- 

 mirror-grating, are alone significant, that two individual light 

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

 appreciably identical. Beyond that the waves no longer cor- 

 respond in orientation and cannot interfere in a way to 

 produce alternations of accentuated brightness and darkness. 

 Again, these 5 millimeter lengths may actually be discontinu- 

 ous and represent successive discharges of radiation (each last- 

 ing about 1 - 5X10"" seconds), separated by absence of light 

 motion along the ray. 



Through the years, I seem still to hear the lament of my old 

 friend and teacher, Ogden 1ST. Rood, of the time he had wasted 

 looking for the interference of differentiated light waves. If 

 with more modern facilities I have reached a conclusion, it 

 would be a privilege to associate it with his memory. 



Brown University, Providence, R. I. 



