C. Barus — Interferences of Grossed Spectra. 493 



centers of the ellipses to which they belong. The appearance 

 is roughly suggested in figure 3. They pass from the type a 

 through h (contraction toward the violet end was not noticed), 

 into the type c, when mirrors m n move in a given direction. 

 The center of the ellipses is in the vertical through the field of the 

 adjustment b, in which case the lines pass from end to end of 

 the spectrum as a narrow band near the longitudinal axis of 

 actual coincidence of spectra, symmetrically. 



^^ A 



The height or breadth of the longitudinal interference band, 

 d, in fig. 3, is not greater than 1"5 to 2 times the distance apart 

 of the sodium lines at right angles to the band. From this the 

 angular divergence of the breadth of the band may be found, 

 since X = D sin 6, where X is the wave-length of light, D the 

 grating space and 8 angle of diffraction. Hence for the two 

 sodium lines Ad = A\/D cos 6. Since D =■■ 351 X 10 -6 , 

 cos = -986, and AX = 6 X lO' 8 , 



A0 = VI X 10~ 4 radians. 



Since the width of the band is about twice this, it will be 68 

 seconds of arc, or roughly about a minute in breadth. Within 

 the strip, when the fringes are horizontal, I counted about five 

 of them, so that their distance apart would be about 14 seconds 

 of arc. 



It appears, therefore, that rays of a given color, say of the 

 wave-lengths at D, which leave the grating at a given point 

 and at an angle of about one minute in the plane of the D 

 line, are still in a condition to interfere ; whereas strictly 



