56 



THE INTERFEROMETRY OF 



lines, visible to the naked eye or through a lens or a suitably strengthened 

 telescope. They decrease rapidly with increase in e, but vanish to the eye 

 before the preceding set in paragraph 22. The sodium lines need not be in 

 adjustment, but the longitudinal axes of the field must be, as usual. If diffuse 

 white light is present, faint colored fringes may be seen at the same time. 

 If the collimator only partly fills the field of view, these diffuse light fringes 

 and the preceding set may occur together. Both rotate markedly for slight 

 rotation of either grating in its own plane. There seems to be a double peri- 

 odicity in the yellow field, but it is too vague to be discerned. When magni- 

 fied with a lens, they admit of a play of e within about 0.6 cm. from contact. 

 When the sodium lines are not coincident, the focal plane continually changes 

 with e. Otherwise it remains fixed. 

 Some data are given in table 7. 



TABLE 7. Ives grating. Homogeneous light. Collimator and slit removed. Focus 



Homogeneous light. Collimator and slit removed, 

 continually changing. 



3eXio 3 =o.95 cm. 



I.OI 



1.04 



1.02 

 0.96 



38 



Large fringes; ocular out; lens on < 



Ocular in, lens on 



Very small fringes, lens off 



Wallace grating. Sodium lines coincident. 



Principal focal plane SeX 10"= i .08 cm. 



1.18 

 i. 20 



These data are similar to the above and subject to the same discrepancy 

 whenever slight variation of the angle of incidence accidentally occurs. In 

 figure 38 the case of three rays from a given 

 flame-point F is shown corresponding to the 

 equation 



_ X cos i 

 i cos (0+0 



when i passes from positive to negative values. 

 If either of the gratings is displaced and if they 

 are parallel, the focal plane will not change; 

 but if G and G' are not parallel, the focal plane 

 differs from the principal plane and now moves 

 with the grating. 



24. Inferences. The above data show that the equation underlying all the 

 interferences observed is the same. The interferences themselves may result 

 from different causes, but their variation in consequence of the motion of the 

 grating, Se, is due to one and the same cause. This is best seen by producing 

 them simultaneously in pairs. As a means of finding an accurate compari- 

 son of the number of lines per centimeter on any grating, in comparison 

 with those on the given grating, the method used in paragraph 20 deserves 

 consideration. 



