64 



THE INTERFEROMETRY OF 



shows the fringes at its edges only. While the spectrum may thus be covered 

 with independent strands of fringes, they all move or change phase together. 

 The range of displacement of the micrometer-screw is about 2 mm. When 

 the spectra are about to separate i.e., when overlapping ceases the fringes 

 are apt to be particularly wide, say even 4DiD 2 , and they may be seen in 

 the gap between spectra. In general, the degree of coarseness of fringes de- 

 pends on the adjustment for parallelism of spectra. 



30. Apparatus and results for inverted spectra. The preceding apparatus 

 may be modified as shown in figure 43 . Here L is a collimated beam of white 

 sunlight from the vertical slit, G a transmitting grating. The component 

 pencils a and a' of spectrum light impinge on the opaque mirrors M and N, 

 and are then reflected in b and b' to 

 reach the silvered sides of the right- 

 angled prism P' . This is now placed 

 with its sides at 45 to the horizontal 

 and its edge in the direction of the 

 incident beam L prolonged. Hence 

 the two pencils b and b' are reflected 

 vertically upward (fig. 44) and appear 

 as two identical and parallel spectra 

 in the field of a telescope, vertically 

 above P'. Observation is conveniently made downward to obviate additional 

 reflection. If the triangle a, a', b', b is horizontal and isosceles there is no 

 difficulty. On slightly moving the adjustment screen on M, N, P' (which 

 must be revolvable on a vertical axis, and P', or the beam L, movable up or 

 down), to bring the two spectra into coincidence along a given longitudinal 

 axis, and a transverse axis like the D lines, the two spectra are symmetri- 

 cal i.e., mirror images one of another with respect to the longitudinal axis. 

 If, now, the mirror M is moved on a micrometer, the fringes of inverted spec- 

 tra appear when the path-differences are nearly the same. 



43 



45 



c 



Obtained at short distances (of the order of a foot) on the cast-iron block 

 (Chap. I, fig. 3), these fringes are quiet and better circumstanced for obser- 

 vation; but their characteristics are the same as found for them before (Chap. 

 I, 4, Carnegie Inst. Wash. Pub. 249). They lie within a narrow strip at 

 the line of symmetry of the two superposed spectra, running from end to 

 end and in breadth about three times the distance apart of the sodium lines 

 (sDiDz). When the mirror M is moved micrometrically in one direction, 

 the fringes begin to appear as fine hair-lines (a, fig. 45) parallel to the D lines. 

 These fine fringes gradually coarsen and rotate until they reach their max- 



