46 



THE INTERFEROMETRY OF 



nearly parallel, but one is always very large in comparison with the other. 

 In case b the difference in size is even more marked and the fringes are nearly 

 orthogonal. In intermediate cases fine large strands occur. These pass into 

 each other continuously; the manner does not admit of description. They 

 are seen best in the principal focal plane and both sets are about equally strong. 



To obtain these fringes the adjustment was first carefully made with the 

 sodium arc. Thereupon the arc was replaced by concentrated sunlight and 

 fine fringes were recognized in the superposed spectra (longitudinal and 

 transverse axes coinciding). These fine fringes were then enlarged both by 

 rotating the grating G' (fig. 25) on its normal axis and readjusting M in each 

 case, and by adding trial compensators in the M or N pencils. A glass plate 

 3 mm. thick gave the best results and they were very striking. The following 

 are the chief characteristics : 



When the mirror M is slowly rotated on a horizontal axis, moving one 

 spectrum vertically, slightly over the other, the fringes pass through all their 

 phases. 



When M is slowly rotated on a vertical axis, which slides one spectrum 

 horizontally over the other, the fringes are displaced, more or less, bodily 

 in the spectrum. Thus in the case b, figure 27, the D doublets are many 



29 



28 



times their own spacing (D } D^ apart. If the two D doublets approach each 

 other, the fringes approach the D line from larger wave-lengths and vice versa. 

 The fringes were lost when the doublets crossed over each other. 



Rotation of the compensator in the first place moves the fringes as in inter- 

 ferometry, as does also the normal micrometric displacement of M. If this 

 motion requires readjustment of M, the range of displacement is curtailed 

 and the corresponding change of phase appears. In the second place, the 

 compensator, on rotation, traces the contours of the curves by successively 

 accentuating the vaguer parts, as will presently be explained. 



The fore-and-aft motion of G' also moves the fringes bodily through the 

 spectrum without marked change of phase. All fringes, whether produced 

 with or without compensators, are ultimately curved lines. 



The most remarkable results occurred on widening the slit. Supposing 

 that large strands were visible in case of the fine slit, and that this was grad- 

 ually widened until the slit width was 0.5 mm. or more, the strands were found 



