116 



THE INTERFEROMETRY OF 



Since the beam of light coining out of the collimator and traversing the 

 grating is a vertical ribbon of light, several centimeters high vertically, but 

 very thin in comparison (a few millimeters) horizontally, it is relative to the 

 vertical plane that the marked effect must be expected. In figure 81, G is 

 the grating, cc the principal plane of the concave, cv that of the convex lens, 

 M the opaque mirror. If the beam consists merely of the axial pencil c, the 

 distorting effect due to the introduction of the lens doublet is slight for any 

 value of their distance apart, D. The two lenses are practically equivalent 

 to a plate. If a broad beam dd is in question and the rays retrace their path, 

 the same is still true. But if, on changing D, the rays do not retrace their 

 path, so that the equivalent lens is convergent or divergent, then the rays 

 after leaving M re-impinge on the grating at different angles than before and 

 the interference pattern is correspondingly changed, principally in its vertical 

 relations. 



Thus it is the lens system which changes the obliquity of rays lying in a 

 vertical plane and passing through the grating, to the effect that the axial rays 

 may represent a case of either maximum or minimum path-difference. The 

 latter will be the case when the divergent pencil which usually traverses the 

 grating becomes convergent in consequence of a sufficiently large value of the 

 D of the lens system. 



81 



83 



66. Observations largely with weak lenses and short interferometer. The 



film grating used (Wallace, 14,500 lines to the inch) was cemented with Canada 

 balsam to a thick piece of plate glass, so that the total thickness of plate at the 

 grating was 1,734 cm. This introduces a large excess of path in one of the 

 component beams; but it is generally necessary, if the stationary interferences, 

 due to the reflection at the two faces of the plate of the grating, are to be obvi- 

 ated and if the ellipses produced are to be reasonably large for adjustment (cf . 

 69). The lens doublet was to be added on the same side as the glass speci- 

 fied, so that the excess of glass thickness on one side was further increased by 

 about 0.19 cm., on the average. Under these circumstances the ellipses were 

 strong, but (in view of the large dispersion) with inconveniently long horizon- 

 tal axes. 



On inserting the doublet (convex and concave lens, each i meter in focal 

 distance) with its concave lens at the mirror and gradually increasing the 

 distance D by moving the convex lens toward the grating, a series of forms 



