G Barns — Interferences of Grossed Spectra. 497 



greater than - 1 mm., are still capable of producing interference 

 in each of the colors of the spectrum (longitudinal axes coin- 

 ciding). This phenomenon is virtually one of homogenous 

 light, the same type of interference occurring in each color 

 from red to violet. They belong moreover to the elliptic cate- 

 gory, being of the same nature as those 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 their light to different 

 sources ; i. e. they are not color edimages of one and the same 

 point in the slit. 



Again, in case of rotation of one of the coincident spectra 

 around a transverse axis (Fraunhofer line), colors which differ 

 in wave length by less than half the distance apart of the two 

 sodium lines also admit of interference. This permissible dif- 

 ference of wave length is thus relatively about 



A A -5 X 6 X 10- 8 



— = — j- = 5 X 10- 4 



X 59 X 10-" 



or less than *1 per cent. The character of these interferences 

 is distinctive. They are not of the regular elliptic type, but 

 usually arise and vanish in a succession of nearly vertical 

 (parallel to slit), regularly broken lines. Later observations 

 however, revealed as their true form a succession of long, 

 spindles or needle-shaped designs. The chief peculiarity is 

 observed in 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 con- 

 ditions of the possibility of observable beating light waves. 

 For two waves very close together of frequency n and n' and 

 wave lengths X and A', if Fis the velocity of light, the number 

 of beats per second would be 



n'— n= V (1 /A.'— l/A) = FAA/A 2 , nearly. 

 Therefore in case of the two sodium lines for instance, 



n'— »j = 3X 10 10 X 6 X 10- 8 /3500 X lO' 13 = 5 X 10' 1 



i. e. about 5 X 10 10 beats per - 1 second, the interval of flicker- 

 ing. Naturally this seems to be out of all question : yet one 

 is confronting a source which is an approach to a mathematical 

 line ; and though I am not apt to stretch a rather conservative 

 imagination quite so far, I should like to see this interference 

 produced under absolutely quiet surroundings. Its 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, unfor- 

 tunately, all too familiar. 



Am. Jour. Sci.— Fourth Series, Vol.XL, No. 239.— November, 1915. 

 33 



