114 C. Bancs — Displacement Interferometer. 



In the case of the figure where the angle of incidence /is 

 small, the distance from M to N is about 70 cm . It may easily 

 be increased to several times this, by inserting longer gas pipes at 

 c and d with appropriate braces. The fringes are very stable 

 even when the instrument stands on a table fastened to a wall 

 bracket. They naturally quiver when the observer is manipu- 

 lating the micrometer screw, but they return at once to quies- 

 cence when the hands are removed. To obviate quivering, i. e. 

 to follow the motion of individual rings, the usual tangent 

 screw method may be employed. 



2. Other Interferences. — The same apparatus may be adapted 

 for observing the linear diffraction-reflection interferences 

 described by Mr. M. Barns and myself.* The equations here 

 available are 



8e = A/2cos£; he' = A/2 cos 0; Be" = X/2 (cos 0-cos i) 



where Be, Be' and Be" are the respective increments of the air 

 spaces between the face (rulings) of the grating and the par- 

 allel opaque mirror in front of it, per fringe passing the cross 

 hairs of the telescope, or a given spectrum line, X the wave 

 length of light and . i and the angles of incidence and dif- 

 fraction in air. For the measurement of Be, \, i, 6 should be 

 known or determinable. 



For these observations let the micrometer G be removed 

 from its plate I and now bolted down on the graduated plate 

 iT(ngs. 1, 2), the table / and appurtenances being discarded. 

 This must be so done that the face of the opaque mirror JV 

 now mounted on the rigid part of the micrometer and the rul- 

 ings of the grating (remounted on the slide of the micrometer) 

 are in the axis of rotation, with the lines of the grating parallel 

 to it and the slit. Hence the mirror must be adjustable by 

 aid of a capsule with set screws (horizontal and vertical axes of 

 rotation) and springs. The grating has its independent mount- 

 ing with three similar set screws and springs. Usually JV will 

 be attached to some apparatus whose linear excursions are to 

 be found, and for this purpose of attachment the cross-coup- 

 ling R in fig. 1 is abundantly supplied with screw sockets 

 (front and rear, not shown), so that such parts may be here 

 secured. A counterpoise, for instance, may be added in the 

 rear. 



To prepare for observation, the plate iTand the telescope E 

 are turned nntil a suitable angle of incidence i and of diffrac- 

 tion 6 are obtained. The fringes are seen when grating and 

 mirror are sufficiently near together (the distance apart may be 

 as much as l cm ), on condition that the direct images of the slit 



* The grating interferometer. Science, xxxi, 394, 1910 ; Phil. Mag. (6), 

 xx, p. 45, 1910; Carnegie Publications, No. 149, chap. 2, 1911. 



