REVERSED AND NON-REVERSED SPECTRA. 17 



used in displacement interferometry. With the exception of the points lying 

 on the longitudinal axis of rotation or of coincidence, all the pairs of points 

 of the two coincident spectra owe the major part of their light to different 

 sources; i.e., the points of the superposed spectra are not colored images of 

 one and the same point in the slit. 



Again, in case of rotation of one of the coincident spectra around a trans- 

 verse axis (Fraunhofer line) , colors which differ in wave-length by about half 

 the distance apart of the two sodium lines seem also to admit of interference. 

 This permissible difference of wave-length is thus relatively about 



AX_. 5 X6Xio- 8 



\ 59X10-0 



or less than o.i per cent. The character of these interferences is distinctive. 

 They are not of the regular elliptic type, but arise and vanish in a succession 

 of nearly vertical (parallel to slit), regularly broken lines. Later observation, 

 however, revealed as their true form a succession of long spindles or needle- 

 shaped designs. The chief peculiarity observed is their almost scintillating 

 mobility, which in the above text has been referred to the inevitable tremors 

 of the laboratory. It is, however, interesting to inquire into the conditions 

 of the possibility of observable beating light-waves. For two waves, very 

 close together, of frequency n and n' and wave-lengths X and X', if V is the 

 velocity of light, the number of beats per second would be 



AX 



Therefore in case of the two sodium lines, for instance, 



w'-n = 3 X io'X6X io-Y 3 48oX io- 12 = 5 X xo 11 



i.e., about sX io 10 beats per o.i second, the physiological interval of nickering. 

 Naturally this seems to be out of all question, even if one is confronting a 

 source which is an approach to a mathematical line. The endeavor will have to 

 be made to produce these interferences under absolutely quiet surroundings. 

 Their appearance is altogether singular and not like the case of paragraph 4, 

 where there is also perceptible tremor, or with the general case of trembling 

 interference patches, with which I am, unfortunately, all too familiar. 



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

 an observation which is spatial, 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. If the coincident path of two rays of slightly 

 different wave-lengths, X and X', which interfere, is %, then there are x/\ and 

 x/\', complete waves in the given path, and, in case of original identity in 

 phase, instantaneous reinforcement will occur when 



x(i/X-i/X') = i,2,3, ...... n 



In other words, at the wth reenforcement 





