60 



THE INTERFEROMETRY OF 



The fringes are easily found and splendid, though they are not superior in 

 this respect to the fringes obtained by other methods. P is also on a microm- 

 eter with the screw in the mean direction b b'. The range of displacement of 

 P was about y = 0.55 cm., so that the available path-difference is considerably 

 above a centimeter. Within this the fringes pass from fine hair-lines through 

 a maximum and back again, apparently without rotation. To enlarge the 

 fringes the grating G may be tilted in its own plane or a similar adjustment 

 made at P. 



The longitudinal axes of the spectra (wire across the slit) are not in focus 

 with the D line; but though hazy they suffice for adjustment. Naturally the 

 distance FbaG is the radius of the grating R (6 feet) and the distance Sm-\-mG 

 is R cos 6. The distances are approximately laid off and the observer at T 

 may then push the mirror M fore and aft by the aid of a lever till the Fraun- 

 hofer lines are sharp. 



Unfortunately the spectra as a whole are shifted by the micrometers, to- 

 gether when P moves and separately when M or N moves. 



27. Polarization. The two rays obtained from calc spar, if corrected for 

 polarization, should be available for interferences of the present kind. Nat- 

 urally an achromatized calc-spar prism (C, fig. 41) is most convenient for the 



purpose. White light from a collimator, L, is doubly refracted by this prism 

 C, and the extraordinary ray a, just missing the grating G (above or on the 

 sides), impinges on the opaque mirror M and is thence reflected to the grating 

 G. The ordinary ray a' is reflected from the opaque mirror N and thence also 

 reaches the grating. These two pencils, b, b', are directly reflected by the 

 grating (DXio 6 = 2oo cm.) into R m and R n and diffracted into D' ro and D' n . 

 By suitably adjusting the grating between C and M and inclining it as shown, 

 two coincident spectra may be made to pass along the common direction c 

 to the telescope at T. These spectra are quite intense. One is somewhat 



