74 



THE INTERFEROMETRY OF 



The phenomenon, which should be observed with a powerful telescope, 

 usually consists of three small elongated dots, lying within an elliptic locus, 

 the locus usually having a transverse axis (parallel to the Fraunhofer lines) 

 about two or three times as long as the longitudinal axis (parallel to lengths 

 of spectra). As a rule, the width was DiD 2 and the length larger than D\D' Z , 

 but this ratio may be changed, as above, by screening off the wave-front. 

 The fringes are not more than one-half of D\D^ apart and are frequently 

 horizontal (longitudinal), however the micrometer at M is shifted. 



The interesting result is here again met, incidentally, that spectra, though 

 of different lengths, are nevertheless quite capable of producing strong inter- 

 ferences. 



50 



a 



>, 



b 



d 



51 



In further experiments with the long collimator and very bright spectra, a 

 variety of other forms were obtained, shown at b, c, d, figure 50. In the patterns 

 a and b, the elliptic outline, sometimes circular, is always evident from the 

 enhanced brightness of the bright fringes of the spot. The arrow-shaped 

 form, c, inclosed a bright egg, whereas d was usually sharply semicircular. 

 As any adjustment of overlapping spectra suffices, the D lines may be quite 

 out of the field, or the spectra may be slightly separated with the interfer- 

 ence spot in the gap. 



The experiment was also made of crossing the spectra at some other angle 

 than o or 180. To do this the rulings of the prism grating were placed at 

 right angles to those of the interferometer grating, as in Newton's method of 

 crossed prisms. Seen in the telescope (adjusted for inverted spectra, as 

 above, 30) the two spectra now made an elbow with each other, figure 51, 

 while the D\D% lines are still parallel and can be put in coincidence. At 

 first no interferences could be detected in any adjustment. Later, however, 

 on using the large collimator, strong interferences were obtained in the line 

 of symmetry of the elbow and normal to the D lines, as shown. They have 

 the same characteristics as the preceding and persist during a displacement 

 of M of about 0.3 cm. 



37. Experiments with the concave grating. If in the device figure 40, 

 Chapter I, the prism P is rotated 90 on an axis parallel to b b', so that the 

 rays move upward, the phenomenon of inverted spectra may be realized. 

 The fringes are observed with a lens from above or reflected forward. They 



