REVERSED AND NON-REVERSED SPECTRA. 101 



Returning to the work with two successive gratings and white light, the 

 channeled spectra obtained are too complicated for concise description. A 

 very interesting result, however, is the passage of the fringes across the sta- 

 tionary sodium line, when the first grating G is moved fore and aft in a direc- 

 tion normal to its plane. The region of the D line is thus alternately dark and 

 bright. The direction of these rays remains unaltered while the illumined strip 

 is shifted horizontally across the ruled space (fig. 70) of the second grating. 

 Usually it is difficult to see the D line in the focal plane of the fringes. When 

 homogeneous light is used this fiducial mark is necessarily absent and the 

 cross-hairs of the ocular must be supposed to replace it. The shift of the 

 fringes is then equally obvious, and sometimes (sodium light) different groups 

 seem to travel in opposite directions while the grating G moves in one direction. 

 In case of homogeneous light and two gratings, moreover, the fringes seem to 

 be of minimum size in the conjugate focal plane of the gratings. They 

 increase in size and in turn split up in focal planes before and behind this. 



An insight into these occurrences was finally obtained in observation with 

 homogeneous light in the spectrometer by shifting the grating (transmitting) 

 in its own plane, right and left. The fringes in such a case move bodily across 

 the field of the telescope, new groups entering on one side for those which 

 leave on the other. These fringes, even if quite distinct, are differently 

 arranged in coarse and fine series and are frequently accompanied by dark or 

 bright bands. This probably also accounts for the effect of the fore-and-aft 

 motion of the grating, mentioned above. Moreover, it would be interesting 

 to search for repetitions of given groups of fringes while the grating is being 

 shifted parallel to itself, from end to end, as this might indicate the residual 

 imperfections of the screw with which the grating was ruled. If the ocular is 

 drawn and set outward from the principal focal plane (at which the slit image 

 is quite sharp) into a different position, the fringes move in a direction opposite 

 to the grating. If the ocular is set inward from the principal focal plane, they 

 move in the same direction as the grating. This would not be unexpected ; but 

 secondary fringes or something else in the field seem to remain stationary. 

 Successive fields may be quite different as to arrangement of fine and coarse 

 lines, but all plane gratings exhibit the same phenomena. Thus it is obvious 

 that the fringes of the present paper result from a residual irregularity in the 

 rulings of the grating. Micrometrically, the successive strips of a slit image, 

 however fine, are of unequal intensity. Between these there is diffraction, as 

 may be tested by examining the clear glass at the edge of the ruled space. 



To attempt a theory of these phenomena seems premature ; but it is obvious 

 that in the otherwise indistinguishable images of a slit in homogeneous light, 

 however sharp or however narrow, the nature of its origin still persists and 

 may be detected by observations outside of the principal focal plane. A fine 

 slit is in all cases presupposed, and all the phenomena vanish for a wide slit. 

 On the other hand, the width of the pencils of parallel rays may be far greater 

 than is necessary to show the strong Fraunhofer lines, if indeed there is any 

 limitation to this width. 



