10 



THE INTERFEROMETRY OF 



a bright line, or a black line between two bright lines, seemed to be visible; 

 but the interferences would have to be stationary to be definitively described, 

 since the width of the pattern is not more than one-third to one-half of the 

 distance between the sodium lines. 1 The interferences, moreover, did not 

 now readily conform to the design B, figure 2, anticipated, but were more of 

 the type C, with long, dark lines slightly oblique to the vertical, and vibrating 

 within a vividly yellow band. Sometimes these were heavier, with two or 

 three faint lines on one side. 



Further experiment showed that the phenomenon is not influenced by 

 the width of the slit, except that it is clearest and sharpest with the narrowest 

 slit possible and vanishes when the slit is made so wide that the Fraunhofer 

 lines disappear. It may easily be produced by the modified method following, 

 in any wave-length red, yellow, green, etc., with no essential difference except 

 in size. It is present, moreover, in all focal planes, i.e., the ocular of the 

 telescope may be inserted or pulled out to any distance, yet the same phe- 

 nomena persist on the vague, colored background. A number of observations 

 were made to detect the change 

 of the pattern of the interference, 

 between its entrance into the field 

 and its eventual evanescence, in 

 case of the continuous displace- 

 ment of the mirror M over 5 mm. 

 In figure 2 this would be equiva- 

 lent to a passage of B into B' 

 through A, and the fringes for a 

 distant center should therefore 

 rotate, as they actually do in 

 the experiments of the next para- 

 graph. But in the present case the type C persists; the lines may become 

 longer or all but coalesce and their inclination may change somewhat. 

 They nevertheless remain fine and nearly vertical, until they vanish completely 

 and there is no rotation. Nor could the phenomenon be found again within 

 the length of the given micrometer screw. Hence it is improbable that these 

 interferences conform at once to the ordinary elliptic type for which figure 2 

 applies, even if the ellipse is considered exceptionally eccentric. The use of 

 two slits, one following the other, does not change the pattern. 



The modified method of experiment was one of double diffraction. In 

 figure 3, L is the blade of light from the collimator, which passing under the 

 plane mirror, m, penetrates the grating G, whence the diffracted first-order 

 beams reach the opaque mirrors M and N. These return the beams, nearly 

 normally but with an upward slant, so that the color selected intersects the 



1 The use of the D\ D 2 distance of the sodium lines for the measurement of the breadth 

 of the interference phenomenon is a mere matter of convenience in describing it. It will 

 be shown in the next report that the breadth of the strip carrying interference fringes is 

 quite independent of the dispersion of the optic system. 



