502 



C. Barus — Use of Compensators. 



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 convergent lens system. 



4. Observations largely with weak lenses and short inter- 

 ferometer. — The film grating used (Wallace, 14500 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 

 T 34 cm . This introduces a large excess of path in one of 



Figs. 1, 2. 



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the component beams ; but it is generally necessary if the sta- 

 tionary interferences, due to the reflection at the two faces of 

 the plate of the grating, are to be obviated and if the ellipses 

 produced are to be reasonably large for adjustment (cf. §6). 

 The lens doublet was to be added on the same side as the glass 

 specified, so that the excess of glass thickness on one side was 

 further increased by about -19^ on the average. Under these 

 circumstances the ellipses were strong, but in view of the large 

 dispersion with inconveniently long horizontal axes. 



On inserting the doublet (convex and concave lens, each 1 

 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 were obtained which 

 passed from the initial horizontal long ellipse, through circles, 

 vertically long ellipses, vertical lines, into hyperbolic forms of 

 increasing excentricity, as recorded in fig. 1. 



On reversing the system, keeping the convex lens fixed near 

 the mirror and increasing the distance D by moving the other 

 lens toward the grating, the. original ellipse usually flattened 

 out further, as shown in fig. 2. Moving the lenses sideways, 



