162 Barus — ZTse of the Grating in Interferomeiry. 



lines or ellipses with reiiiote centers are cine to sininltaneous 

 rofloctions of the component rays from the opposite faces of 

 the grating, so that the angle of the wedge of glass cannot be 

 exclnded. All owe their vertical and horizontal symmetry to 

 the vertical slit and horizontal spectrum. Tlie ellipses are 

 identically present in the successive orders of spectra at once. 



These elliptical fringes thus embody with the preceding 

 (1. c.) the connnon property of being duplex in character ; only 

 here the motion of dark rings to or from the centers of the 

 ellipse, as a fine adjustment, is associated with a displacement 

 of the fringes bodily through the spectrum (coarse adjustment). 



This displacement may be from red to violet or from violet 

 to red as the virtual air-space increases in thickness, depending 

 upon the adjustment, as will presently appear. In other worjis, 

 for a given small interferometer motion of mirror, there is in 

 general less displacement of fringes bodily than radial motion 

 of fringes to or from the center. Slight deflection of the 

 mirror is chiefly accompanied by radial motion of the fringes.* 

 The effect thus produced is to give sharpness to the interfer- 

 ence pattern, which soon vanishes for approximate adjustments 

 of spectra. 



Similarly the micrometer screw produces a rapid passage of 

 the pattern through its maximum size, a play of the screw 

 of -l*^" being sufficient to pass from fringes of one extreme 

 of small size, through the maximum size to the final extreme. 

 On the other hand, there are many regions of interference ; in 

 fact, different groups of interferences may sometimes be in the 

 field at once, or more usually corresponding to slightly differ- 

 ent angles between the mirrors and grating. 



All admissible angles, provided that symmetry is maintained 

 (the virtual plane of the grating bisecting the angle of the 

 mirrors), bring out the phenomenon. This points to the fact 

 that the earlier phenomenon referred to (1. c.) in which the 

 center or normal ray was virtually at an infinite distance, is 

 now produced near an accessible center, even if this is not 

 actually in the field. However, in the earlier case when the 

 angle of incidence is zero, curved lines may also be obtainable. 



Experiments with a silvered grating showed no advantage, 

 whether the transparent film covered the grating or the plane 

 face of the glass. In fact in case of good adjustment the phe- 

 nomenon is so strong as to need no accessory treatment. This 

 is, in fact, one of its advantages. A great difficulty in adjust- 

 ment is the occurrence of stationary fringes due to the rear 

 face of the grating. These may even wipe out the spectrum 

 lines; but usually they lie at a finite focus and are not seen 



* This changes the effective thickness of the grating. 



