416 



C. Barus — Methods in Reversed and 



Whence 



and 



<f> = 18° 13', 8= 36° 6', 8 = 17° 54', 



•50 X -1556 

 X = - - = -OOG 01 " 



•808 



if e = ^S 01 ", as found. Thus the rays of the same origin, or 

 rays capable of interfering, are found in a vertical strip on the 

 grating not more than 1 millimeter wide. It is interesting to 

 note that the fringes vanish by becoming coarser and wider 

 corresponding to the narrowing of effective edges in contact. 



The attempt to produce these fringes with homogeneous 

 (sodium) light and a wide slit again failed, although much 

 time was spent in the endeavor. Even with a narrow slit and 

 accentuated sodium lines (impregnated arc), the phenomenon 

 may be produced between the doublets, however close together, 

 but it fails to appear with the same adjustment when two cor- 

 responding lines coincide. I was only able to produce it in a 

 continuous spectrum between the two doublets, and with a fine 

 slit. It is important to ascertain the reason. 



Both mirrors, M and JV, are now placed on micrometers 

 moving nearly normal to their faces. Beginning with a coin- 

 cidence of the illuminated strips on the grating, the M mi- 

 crometer was moved until the fringes disappeared. The JV 

 micrometer was then moved in the same direction, until the 

 reappearing fringes passed through an optimum and finally 

 vanished, in turn. Thereafter the M micrometer was displaced 

 again, always in the given direction and the same cycle re- 

 peated ; etc. It was possible to pass through about 8 cycles 

 with each micrometer, before the illumination reached the 

 edge of the grating, each cycle corresponding to a displace- 

 ment of about 2 millimeters for a single mirror ; but a total 

 displacement of 2-5 em was registered which would obviously 

 have been increased much further if the grating had been 

 wider. The following data gives a concrete example : 



Position 



Advance 





Position 



Advance 



of N 



of N 



Eemarks 



of iV 



of N Eemarks 



2-42 cm 

 2 20 







Broad 

 arrows 



l-75 cm 

 1-54 



v Vertical lines 



- - - - 



•22 cra 



M advanced 



- - _ - 



•21 cm M advanced 



2-20 

 1-98 



::::• 



Narrow 

 arrows 



1-54 



1-37 



>• Vertical lines 







•24 



M advanced 







1-7 M advanced 



T98 

 1-75 



.... j 



Upright lines 

 inclination 



etc. 





.... j 



•23 



changed 

 M advanced 







