34 THE INTERFEROMETRY OF 



of the grating. It was not at all difficult to follow the continuous tilt of these 

 lines through the horizontal, occurring on careful and continuous front-and- 

 rear motions of the grating G' through the limiting positions. The fringes 

 usually vanish vertically merely because of their smallness. 



Again, on rotating the grating G' around an axis normal to its face, the 

 fringes merely vary in size, without changing their inclination. Thus if the 

 horizontal fringes (which were here always closer than the inclined set) are 

 in view, these will pass from extremely small-sized, fine, hair-like striations, 

 through a maximum (which is a mere shadow, as a single fringe probably 

 fills the field) back into the fine lines again. Only a few degrees of rotation 

 of the grating suffice for the complete transformation. The maximum is 

 frequently discernible only in consequence of a flickering field. An oblique 

 set of fringes is equally available, remaining oblique as they grow continually 

 coarser and in turn finer with the continuous rotation of the grating. 



When the very large horizontal fringes are produced by this method, the 

 change into vertical fringes by fore-and-aft motion of G' is very rapid, so 

 that relatively wide, nearly vertical forms may be obtained. All these effects 

 may be produced by solar or by arc light, around 

 the line of symmetry of the overlapping spectra; 

 or with sodium light when either DiD'i or D 2 D'z 2 1 

 coincide. 



The fine vertical or inclined lines appear as & 6 



such when the slit is widened, either in case of 

 white or of sodium light. These are the inter- O 2 

 ferometer fringes seen above ( 6), coarse or 

 fine. With sodium light any width of slit, or 

 no slit at all, is equally admissible. The same is true for the narrow maxima. 

 Lines nearly horizontal were sometimes obtained, pointing, as a whole, toward 

 a center. 



Finally (and this is the important result) the extremely large horizontal 

 maxima, when a single fringe fills the field, can not be seen apart from pulsa- 

 tions, in the case of a wide slit. With a very narrow slit, such as is suited for 

 the Fraunhofer lines, these horizontal fringes appear as intensely bright or 

 very dark images of the slit. In other words, the normal phenomenon of 

 overlapping symmetrical spectra as described in Chapter I is merely the 

 vertical strip of an enormous horizontal interference fringe, made sharp and 

 differentiated by its narrowness. This case occurs at once when the rulings 

 of the two gratings G and G' are all but parallel, and hence it is the regular 

 phenomenon when but a single grating is used for the two diffractions, as in 

 figures ii and 12. 



In later experiments on the effect of the rotation of the grating, G', around 

 a normal axis, the above results were found to be incomplete. If the rotation 

 is sufficient in amount (a few degrees, always very small), it appears that, 

 after enlarging, the fringes also rotate. But the rotation in this case corre- 

 sponds to a vertical maximum, as indicated in figure 22, the vertical set being 



