14 



THE INTERFEROMETRY OF 



Since Z) = 35iXicr 6 , cos # = .986, and AX = 6Xicr 8 ; therefore &9=i.'jXio- 4 

 radians. Since the width of the band is about twice this, it will be 68 seconds 

 of arc, or, roughly, about a minute in breadth. Within the strip, when the 

 fringes are horizontal, I counted about five of them, so that their distance 

 apart would be about 14 seconds of arc. 



It appears, therefore, that rays of a given color, say of the wave-length at 

 D, which leave the grating at a given point and at an angle of about one 

 minute in the plane of the D line, are still in a condition to interfere ; whereas 

 one would anticipate that only those rays which lie in the common longitudinal 

 axis of rotation of the two coincident spectra, symmetrical to this, should be 

 in this condition. Such interference should not be appreciable, since the 

 white rays are independent and apparently come from two different points 

 of the slit. If we consider the angular deviation of pencils of parallel rays 

 crossing the grating to be equivalent to the divergence of their respective 

 optical axes at the collimating lens (about 45 cm. in focal distance), the dis- 

 tance apart of two points of the slit, the rays of which are still able to produce 

 interference, is 



4 5X i. 7X10-^= 7.6Xio- 3 cm. 



or nearly o.i mm. Hence points of white light in the slit about o.i mm. apart 

 along its length are included in the band of interferences in question, extending 

 in colored light from red to violet. This seemingly anomalous result will be 

 fully interpreted at another opportunity. 



5. Interference of the corresponding first=order spectra of the grating, in 

 the absence of rotation. This apparatus seemed to be of special interest, 

 since the rays used do not retrace their path and are thus available for experi- 



ments in which rays traveling in one direction only, are needed.* I have 

 tried both the adjustments given in figures 7 and 8, the latter, since the rays 

 are more nearly normally reflected at the mirrors M and A T , having some 

 advantages ; but the other succeeds nearly as well. The difficulty encountered 

 is a curious one of adjustment, which was not anticipated. In other words, 

 if the longitudinal axes of two identical spectra are in coincidence, the Fraun- 

 hofer lines are likely to be at a small angle to each other and complete inter- 



* Cf. Am. Journal of Sci., xxxiv, p. 101, 1912, on the interferometry of an air column 

 carrying electrical current. 



