Barus — Use of the Grating in Interferometry. 165 



of mirror (micrometer screw) does not much exceed 1""° for 

 ordinary magnification. Tlie second equidistant adjustment 

 brings out the lines or ellipses with distant centers, and there 

 are usually three types easily found (after one has appeared), 

 by slightly inclining either mirror around a horizontal axis by 

 a tangent screw. Thus as seen in fig. 2, fine lines, say 

 135° to the horizontal, coarse, nearly circular lines, concave 

 downward or upward as the case may be, and finally broad 

 lines, say 45° to the axis, may appear in succession. If the 

 grating is rotated 180° around its normal, convexity upward 

 changes to concavity upward, showing the wedge angle of 

 the glass plate to be effective. The micrometer screw will 

 pass any of these forms through a succession of inclinations 



Fig. 2. 



corresponding to the eccentric intersection of a group of con- 

 centric circles by a straight line. This is also shown in fig. 2 ; 

 at A and B as observed for a fixed air space and at G on 

 moving the mici'ometer in the A and B cases. These figures 

 cover the coincident spectrum fields only. A solitary part of 

 the field shows no interferences. For adjustment it is, there- 

 fore, necessary to cut off the spectra sharply above and 

 below by limiting the length of slit. Of the three or four 

 spectra present the coincidence may then be secured with least 

 diffieulty. With each such coincidence there goes a definite 

 position of the micrometer screw (interferometer), and this is 

 the initial difficulty of adjustment ; i. e., to coordinate the 

 various spectrum coincidences with the three screw readings. 



The non-equidistant case of mirrors and grating corresponds 

 to the ring types with tlie centers in the field. Hence both 

 reflections take place from the same face of the grating. When 

 the distance GM \^ shorter, 6^ 'iV longer, I have noticed two or 

 even three ellipses successively in the field, obtained by slowly 

 inclining the mirror JV about the vertical (tangent screw), with 

 their centers in turn in the 3'ellovv-green, the blue and the green 

 of the spectrum. They do not occur together. On turning 



