306 



C. Barus — Use of Compensators. 



two spectra are usually not so far apart. A' corresponds to a 

 virtual object behind the observer. 



If the vertical plane at which the interferences lie be taken 

 as the image, the object would be situated about 3 meters 

 beyond the objective of the telescope used. This would place 

 it about 30 cm in front of the mirror M or _ZT, where there is but 



Figs. 4-7. 



Jt 



($ia. 7. 



cfl 

 a 



a' 

 b' 



6 



cM 



oha. 6. 



a single beam in each case. In fact, the telescope may be 

 brought quite up to the grating. Hence interference is pro- 

 duced in the telescope itself, where rays are relatively very 

 divergent, a condition which accounts for the smallness of the 

 interference pattern. This understanding of the case is tenta- 

 tively shown in fig. 6, where O is the objective of the telescope, 

 M the larger image from the mirror with the lens compensator 

 and JY the image from the other mirror (parallel rays). If the 



