PHYSICS: C. BARUS 
151 
Hence it appears that horizontal fringes are left unchanged when the plane 
of dispersion is horizontal (edge of prism vertical), which is to be expected; 
for 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 
vertical, the nearly horizontal fringes have to pass from the positive to the 
negative inclination through their maximum size, when the telescope is rotated 
over 180°, and hence A iV is very large, particularly so when the fringes are 
relatively small. In this large displaicement of mirror (AiV = .10 cm., nearly) 
small monochromatic fringes will not change their inclination much; but their 
size will change considerably, and thus at A<^ = 90° they are large and at A<^ 
= 270°, small. 
Exactly the opposite conditions are met with when the fringes are nearly 
vertical, and the data may be omitted here. In figure 2, V lies somewhat below 
F, 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. The vertical fringes were larger and hence a smaller double 
ampHtude of displacement (A iV = .07 cm.) was here recorded. 
Finally in the results for achromatic fringes at about 45° (estimated by the 
eye) the maxima were somewhere near A<^ = 45° and 135°. Though the 
results were less smooth here, from deficiencies in the orientation (45°) of the 
achromatics, there was no fault to be found with the clearness of fringes, or 
with their abrupt evanescence. 
If the spectrotelescope Tg (fig. 1) with a very fine slit is rotated, the fringes 
remain parallel to the length of the spectrum passing through a symmetrical 
case where the spectrum is reduced to a single fine colored line parallel to the * 
slit. The fringes remain nearly parallel to the edge of the prism. Hence if 
any form coincides with the achromatic or monochromatic fringes, it will be 
retained on opening the slit wide, whereas the other forms, inasmuch as they 
require a fine slit, will vanish with the Fraunhofer lines. In the absence of a 
slit, the whole colored field bursts into sharp fringes, whenever the proper 
angle A<p of the telescope is reached. If the slit is a little too broad to show 
the solar lines distinctly, the monochromatic fringes may often be detected 
crosshatching the vague Fraunhofer lines, even when the spectrum fringes are 
still strong. 
If the fringes of a fine slit are at say 45° to the axis, their inclination will 
change to 135° on passing the symmetrical stage; but there is apt to be both 
a change of size and angle in such cases. 
Summary. — It has been shown in the experiments that the fringes (mono- 
chromatic) due to differences of inclination of rays, and the fringes (disper- 
sion) resulting from differences in wave length of rays may be made of nearly 
equal size by displacing any mirror of the rectangular interferometer normal 
to itself (A N). The fringes will not, however, generally have the same incli- 
nation. This may be imparted to the spectrum fringes by rotating the spec- 
