REVERSED AND NON-REVERSED SPECTRA. 65 



imum size c, when they are perpendicular to the D lines. A single black or 

 bright line may here extend from end to end of the spectrum. Thereafter 

 they grow finer (rotating again) in the same way until they vanish, e, as 

 they began. The total angle of rotation is thus 180. 



If the two beams b and b', figure 43, are moved nearer the edge of the 

 prism, the fringes become larger, but usually not much. At least I did not, at 

 first, succeed in separating the fringes much beyond the DiDz distance. The 

 width of the longitudinal interference strip remains unchanged. If the light 

 is removed at the line of symmetry (wire across slit), the fringes are sharply 

 outlined across the black line. Being so small, they are naturally always 

 sharp and vivid. The range of displacement of M within which fringes are 

 visible was about 0.2 cm. for the given grating (D = 352 X lo" 6 ), corresponding 

 to the complete rotation 180, instanced above. If the slit is widened, the 

 fringes slightly outlast the Fraunhofer lines. They also lie in the principal 

 focal plane. 



The remarkable feature of these fringes is the definite breadth of strip, 

 from red to violet, within which they lie. With full wave-fronts the stria- 

 tions look as though they were cut between two parallel lines, 3 A A apart, 

 In some adjustments, however, suggestions of concealed fringes, very faint 

 prolongations of the strongly marked striations, are unmistakable. 



31. Wave=fronts narrowed. The longitudinal strip within which the 

 interferences lie is very sharply limited in breadth, as has been stated. It 

 may, however, be broadened by screening off the white beam, as it leaves the 

 collimator, from below. When the whole length of slit is utilized, the strip 

 may not be more than DiD 2 in width. As the vertical blade or beam of white 

 light is cut off, more and more from below, the strip increases to a width of 

 4DiD 2 , and when the two inverted spectra in the field of the telescope begin 

 to separate at the line of symmetry, the strip may be over ioDiDz in width. 

 It is obvious that in such a case the light comes from near the horizontal top 

 edge of the prism. The wave-fronts are slit-like. The field within which 

 diffraction is perceptible in the telescope increases in breadth as the colored 

 wave-front, incident at the objective of the telescope and parallel to the 

 spectrum, decreases in breadth in the same direction. In figure 46, a and b 

 are the two component beams from the two faces of the prism, respectively. 

 The focus is at /. The arrows show the effect of narrowing. The oblique 

 rays (omitted) are similarly affected and in step with the axial rays shown. 

 But the fringes are not changed in size. They may, however, be definitely 

 changed in inclination. Size results from the anterior relations of the spectra 

 (distance between paired pencils), and not from the width of wave-front. 



These inverted fringes admit of much magnification. With a strong tele- 

 scope (magnification about 15) they are quite sharp only in a part of the 

 magnified spectrum and grow vague beyond, showing that the component 

 spectra are not quite identical after the two reflections. When not quite in 

 adjustment, the strip is liable to exhibit separate oblique strands, lying within 



