1 70 Barus — Use of the Grating in Interferometry. 



plane-parallel glass jilates, one of which carries the grating, 

 the ideal ])lane in (piestion is ])rovided. The ellipses in this 

 case wonld he infinite in size and their displacements infinitely 

 large. By partial compensation (compensator thinner) ellipses 

 of any convenient size and rate of displacement may therefore 

 be provided at pleasure. The following table gives some 

 rough experimental data where z is the advance of the microm- 

 eter screw normal to the grating, i. e., the displacement of 

 the grating to move the center of fringes from the D to the h 

 lines of the spectrum, e' the thickness of the compensator 

 plate parallel to tlie mirror J/ or N as stated. 



Displacement of ellipses from the D to the h line, by moving the 

 grating A3 cm. normal to itself. ^■=45°, nearly. Grating: 

 c=-68cra.: D = -000,351 cm.; ^x. =1-5.3 (estimated). Com- 

 pensation shown by negative sign. Position p taken while 

 the sodium line was the major axis. 



Compen- 

 sator 



thickness 

 e'xlO^ 



Position 



of 

 grating 



P 



Micro- 

 meter 

 displace- 

 ment 

 AzxlO-" 



No of 

 fringes 

 Z) to 6 



Displace- 

 ment per 



fringe 

 10«xd2/dn 



Ellipses 



o 



cm. 



— 48 



— 44 



— 29 



— 10 

 -f48 



— 65 

 ± 



cm. 



•05 . 



•08 



•12 



•20 



•38 



-00 



cm. 



30 

 35 

 45 

 65 

 115 

 10 

 70 



12 



44 

 4-5 



28 



cm. 

 2-5 



2-6 

 2 "3 

 2-5 



Very large 

 do. 



Large 



Mean 

 Very small 

 Enormous 



Mean 



N 

 N 

 N 

 N 

 M 

 N 

 None 



Rotation of the compensator offers the usual easy method of 

 adjustment. In the second order the first rings, on compen- 

 sation, usually more than fill the field. This is of course even- 

 tually the case in the fii'st order also. With very perfect com- 

 pensation the ellipses are quite eccentric and the opposed lines 

 nearly vertical and straight. Hence their motion, partaking of 

 the twofold character specified, is complicated but usually 

 opposite in direction on the two sides of the center for the 

 same micrometer displacement. The whole phenomenon may 

 vanish within a half mm. of play of the grating, passing from 

 fine lines through enormous ellipses back into reversed fine 

 lines, all nearly vertical. 



The displacement of the grating by the micrometer screw is 

 the same per fringe, no matter whether the ellipses are large 



