16 THE INTERFEROMETRY OF 



of the possibility of interference; for in neither case ought they to occur if 

 the spectra are not quite coincident horizontally and vertically. If they do 

 occur, it would at first sight seem that a certain small latitude of wave-length 

 adjustment is permitted even with light-waves. 



I was at first inclined to refer the cause of this lack of simultaneous parallel- 

 ism to the grating itself, as it occurred with an Ames grating ruled on glass, 

 with a Michelson reflecting grating, and with a film grating, in about the 

 same measure. But subsequently, on adopting the method of figure 8, the 

 divergence was largely removed and the interferences were now visible 

 throughout the whole of the spectrum. The discrepancy is probably due to 

 insufficient normality of the plate of the grating to the incident white ray, 

 since one of the rays is twice reflected. In any case the adjustment of the 

 coincident sodium lines must be very accurate if the fringes are to be sharp ; 

 certainly as little as half their distance apart will obscure the phenomenon. 



Though the spectra are bright, the interferences are not as good as with 

 the usual method (paragraph i); i.e., the dark lines are not black. Neither 

 have I found an available or systematic method for centering the fringes, so 

 that the lines obtained are usually delicate. Again,. the position of the colli- 

 mator, both as regards slit and lens, is here of very serious importance. Any 

 micrometeric horizontal motion of either, in its own plane, will throw the 

 fringes out. Finally, the whole spectrum travels with the motion of the 

 micrometer mirror M. The apparatus is thus too difficult to adjust for use, 

 to be of practical interest when simpler methods are at hand. The effect of 

 tremors acting prejudicially on so many parts is exaggerated. 



6. Conclusion. The phenomena of paragraphs 2,3, and 4, showing definite 

 and characteristic interference in case of two coincident spectra crossed either 

 on a longitudinal or transverse axis, represent the chief import of the present 

 chapter. These results can not be directly due to the diffraction of a slit 

 (regarding the line of coincidence as such), owing to their relatively small 

 magnitudes and their independence of the breadth of the slit. Since there 

 is in each case but a single line of points or axis, the disturbance of which 

 comes from identical sources, we might regard the image of this line in the 

 telescope to be modified by the diffraction of its objective. But if the inter- 

 ferences originated in this way, the Fraunhofer lines of the spectrum should 

 show similar characteristics and the diffraction pattern should differ from 

 those observed. Thus the conclusion is apparently justified that distinct and 

 independent points of the narrow slit whose distance apart on its length is 

 not greater than o.i mm. contribute rays to the field of interference in each 

 of the colors of the spectrum (longitudinal axes coinciding). 



The phenomenon of inversion is virtually one of homogeneous light, the 

 same type of interference occurring in each color from red to violet. When 

 the fringes are horizontal, homogeneous light and a correspondingly broad 

 slit would replace the spectrum. They belong, moreover, to the elliptic 

 category, being of the same nature, apart from their limitations, as those 



