130 



BARUS— ELLIPTIC INTERFERE^CE 



r April ?i. 



Finally if for any suitable value of c the grating g' is moved in 

 its own plane without rotation away from g, so as to widen the 

 crack at 6^ between them, the fringes grow continually finer until 

 they pass beyond visibility, and vice versa ; i. e., as the crack at 6^ is 

 made smaller the lines continually coarsen. 



5. Nature of the Evanescence. — The fact that the lines vanish 

 as a whole and almost suddenly after reaching their maximum dis- 

 tance apart is very peculiar, as is also the fact that they cannot be 

 passed through infinite size or appear symmetrically on both sides 

 of this adjustment. To investigate this case I provided both the 

 collimator and the telescope with slits so that the parts of the grat- 

 ing g and g', from which the interfering pencils come, might be 

 investigated. 



If a single vertical slit about i mm. wide is passed from right 

 to left toward the objective of the telescope, a black line passe.= 



Fic. 4. Diagram. 



across the field of the spectrum, which line is merely the image of 

 the crack at 5". In the diagram Fig. 4, the G rays, for instance, conif^ 

 from the edge of both gratings g and g' , whereas the R rays an^' 

 the [' rays come from but a single grating. Now when the space e 



