98 



DISPLACEMENT INTERFEROMETRY BY 



73. Observations. For the present purposes the case of achromatic fringes, 

 horizontal, vertical, and at about 45, respectively, will suffice. Moreover, 

 relatively small fringes, requiring much larger displacements (AN) than very 

 large fringes, will generally be preferable. 



Figure 96 gives the results for vertical achromatic or monochromatic fringes, 

 the ordinates showing the displacement of micrometer AN (at M fig. 95) in 

 io~ 3 cm. and the abscissas the corresponding rotation of spectro-telescope, 

 gT, needed to produce sharp fringes in the spectrum of an indefinitely wide 

 slit. When the fringes are small a few degrees of excessive rotation A<p, either 

 way, will cause them to vanish completely, so that the orientation for sharp 

 fringes is quite sensitive. The symbols H (horizontal) and V (vertical) refer 

 to the orientation of the edge of the prism or the lines of the grating. The 

 plane of dispersion is thus normal to H and V. 



Hence it appears that vertical fringes are left unchanged when the plane of 

 dispersion is vertical (edge of prism horizontal), which is to be expected; for 



0' 90 480" W 280 







in such a case the light is permanently absent at the absorption bands due to 

 the inclination fringes. On the other hand, when the plane of dispersion is 

 horizontal, the nearly vertical fringes have to pass from the positive to the 

 negative inclination through their maximum size, when the telescope is rotated 

 over 1 80, and hence AN is very large, particularly so when the fringes are 

 relatively small. In this large displacement of mirror AN = 0.070 cm., nearly, 

 small monochromatic fringes will not change their inclination much ; but their 

 size will change considerably, and thus at A<p = <)o they are large and at 

 A<p = 2'jo small. 



Exactly the opposite conditions are met with when the fringes are nearly 

 horizontal, as in figure 97. In this figure V lies somewhat below V, as I could 

 not (for incidental reasons) obtain adequately horizontal fringes without 

 extreme difficulty. But this amounts merely to a slight shift of phase in the 

 diagram, which is otherwise the counterpart of figure 96. The fringes were 

 smaller and hence a much larger double amplitude of displacement (AN = o. i 

 cm.) is here recorded. 



Finally, figure 98 gives the results for achromatic fringes at about 45 

 (estimated by the eye) , the curves a and b referring to small fringes, whereas 

 c corresponds to large fringes. The maxima are somewhat near A<p = 45 and 



