REVERSED AND NON-REVERSED SPECTRA. 63 



inverted or direct. Hence with a wide slit, or insufficiently homogeneous 

 light in the direction of a given Fraunhofer line, no fringes will appear. Such 

 fringes as may in any case be found will not, therefore, much outlast the 

 Fraunhofer lines, so far as width of slit is concerned. Furthermore, even if 

 the slit is narrow and the spectra coordinated, there will be no fringes obtain- 

 able if the superimposed solar spectra are quite unbroken i.e., without in- 

 cidental furrows in the direction of their length (normal to the Fraunhofer 

 lines). For it is to be noticed that the slit is horizontal, and therefore there 

 is no observable diffraction i.e., virtually no slit in the horizontal direction. 

 If, however, the spectrum field is interrupted longitudinally by a thin 

 (o.i mm.) wire drawn across the slit (or, much better, by very fine specks of 

 dust lying incidentally within the slit), then these fine opaque objects will 

 effectively replace the slit, or act analogously to a slit in the horizontal di- 

 rection. Hence fringes will appear when the path-difference is sufficiently 

 small, associated with the geometric shadow of the opaque objects, through- 

 out the length of the spectrum. 



42 



2T 



We have here, therefore, a peculiar case of the diffraction of a rod, from 

 two separately controlled half -wave-fronts. The fringes at one extreme of 

 adjustment of mirror M begin with fine horizontal lines, which on moving M 

 incline and enlarge until they gain the maximum of size in the vertical direc- 

 tion. After this, on further motion of M in the same direction, they incline 

 further, diminish in size, and finally become horizontal and hair-like again. 

 They move along the horizontal axis with M, subject to the equation 



de X 



dn 2 cos 5/2 



where 8/2 is the angle of incidence at M, X the wave-length, and de/dn the 

 normal displacement of M per fringe. 



Within the region of overlapping spectra each longitudinal black line is 

 covered from end to end with fringes, the strip being about of DiD? width 

 or more if the line is thinner. A broad black line (thicker wire across slit) 



