28 



THE INTERFEROMETRY OF 



a 



slide carrying M is to be moved fore and aft until the interferences appear. 

 These operations are difficult even to an experienced observer. The fringes 

 are very susceptible to tremors, and only under quiet surroundings do they 

 appear sharply. At other times they move, as a whole, up and down and 

 intermittently vanish. 



The fringes so obtained, figure 17, were totally different from the preceding 

 and consisted of short, black, equidistant, nearly horizontal lines across the 

 active yellow strip of spectrum, at the axis of coincidence. The strip was 

 about of the same width as above. Thus the pattern presented the general 

 appearance of a barber's pole in black and yellow, the width being less than 

 the sodium interval, D\, D 2 , and the distance apart of fringes usually smaller. 

 They were visually in motion up and down, rarely quiet, no doubt owing to 

 tremor. Since the fringes were nearly horizonta 1 or less than 30 degrees in 

 inclination, it was possible to enlarge the width of the slit without destroying 

 them, as in case of the hair-like vertical fringes in paragraph 2 above. In 

 this way a breadth of strip greater than the distance Di, D 2 , could be obtained 

 with sunlight or arc light, though a moderately fine slit was still desirable. 



34 



b 



d 



17 



e 



f 



In general, the characteristics noted above were again observed. Thus on 

 moving the micrometer screw controlling M, the interferences appeared rather 

 abruptly. They vanished in a similar manner, after about 0.4 cm. or more 

 of the micrometer screw had been passed over. In other words, the fringes 

 remain identical for a path-difference of about 2X0.4 cm., or nearly 15,000 

 wave-lengths. 



If we call the four D lines available in the two solar spectra DI, D 2 , D\, D' 2 , 

 respectively, a number of curious results were obtained on placing them 

 variously in approximate coincidence. Thus figure 17 a, when each D line 

 of one spectrum coincides with the mate of the other (Di, D'$\ D'i, D 2 ), equi- 

 distant dots, surrounded apparently by yellow luminous circles, appeared 

 between the two doublets. On widening the slit the dots changed to a grating 

 of nearly horizontal lines covering the strip DI, D 2 , figure 176. The lines in 

 one part of the slit seemed to slope upward and in another to slope downward. 

 With a large telescope the phenomenon was more dim and quiet, apparently. 

 The fringes often lie in more definite focal planes and cease to be visible when 

 the ocular of the telescope is far outward, differing from the case above. 



The phenomenon of chief interest, however, was observed (figure 1 7 c] in 

 placing two identical D lines in coincidence (D\; D 2 D' 2 ; D\}. The fringes 



