146 INTERFEROMETRY OF SPECTRA. 



out clearly if the last equation for d is used. In case of corresponding incre 

 ments dd and 8 (A 0), this equation is equivalent to 



bR 



But if L is the length of the telescope and 8n the micrometric displacement of 

 fringes in the ocular corresponding to dd, 



5n 



L 



whence 



5n 



bRL 



Thus if e, the difference in thickness of the corresponding half-silver mirrors, 

 and dn be each even as large as o.i mm., and d a kilometer, 



d=io 5 cm., e=io" 2 cm., b = 200 cm., .R=iocm., L = 5ocm., 5w=io~ 2 cm. 



io 10 Xio- 2 Xio- 2 

 od= - - = io cm. 



200X10X50 



With these very moderate estimates a distance of i kilometer should be 

 measurable to 10 cm., so far as the glass-paths of the interferometer are 

 concerned. 



