128 



BARUS— ELLIPTIC INTERFERENCE 



[April 21, 



while the grating (either transmitting or reflecting) must be at the 

 center of the spectrometer, if angles are to be measured. The same 

 is true for any of the other superimposed white slit images in the 

 above or the earlier experiments and may even be repeated with 

 successive transmitting gratings. It is interesting to note that the 

 position of the center of ellipses is at the same w^ave length in all 

 the spectra though the form of ellipses may differ enormously. The 

 same phenomenon may thus be seen by a number of observers at the 

 same time, each looking through his own telescope. 



lU 



it'-7, 





>^r 



3 



3) 'J>' 



Fig. 2. 



Fig. 3- 



Diagrams showing position of gratings, g, g'. 



4. Third Method. Parallel Gratings. — In this case the two 

 halves of the grating are treated displaced parallel to themselves, 

 from their original coplanar position in the grating, from which 

 they are cut. Their mounting is thus something like the case of the 

 two black plates of Fresnel's mirror apparatus, one of the plates 

 being adapted for displacement parallel to itself. 



In Fig. 2 g and g' show the two halves of the grating cut along 

 the plane S, normal to the plates and parallel to the rulings. The 

 half g' is provided with a micrometer screw, so that it may be suc- 

 cessively moved from the position g' in Fig. 2 tt^ the position g' in 

 Fig. 3, through all intermediate positions, while the half g remains 

 stationary. Each of the halves g and g' is controlled by three ad- 

 justment screws (horizontal and vertical axes of rotation), to secure 

 complete parallelism of the faces of the grating. Each, moreover, 



