98 EXPERIMENTS WITH THE DISPLACEMENT INTERFEROMETER. 



the number desirable for interferometry, as the ellipses when the number of 

 lines exceeds 10,000 to the inch are liable to be too broad for accurate measure- 

 ment. Otherwise the grating sufficed the required purposes admirably. The 

 great advantage in adjustment in case of such a grating is the presence of 

 only one strong reflection, namely, that from the uncovered face of the glass. 

 The side covered by the grating reflects but very feebly. Consequently the 

 grating is to be mounted with the rear side (smooth glass) toward the source 

 of light and the celluloid side toward the eye of the observer. Hence if the 

 two strong slit images in the telescope of the interferometer are placed in 

 contact horizontally and vertically, the ellipses are found for approximately 

 equal distances, without difficulty. They are centered, since both reflections 

 occur at the same position of the same face. It is also possible to obtain faint 

 reflections from the grating face, the slit image being usually deep blue in 

 color. In spite of this, however, the spectrum (as the ray does not again pass 

 through glass) is strong. The interferences are rarely centered, as the two 

 reflections contain the angle of the faces of the glass plate between them. 

 They consist of lines coarsening and rotating 180, as the vertical projection 

 of the distant center is passed. The ellipses, if too broad, may as usual be 

 made smaller with a thick compensator, but at a sacrifice of sensitiveness. 

 With concave mirrors, on the horizontal pendulum for instance, the ellipses 

 are apt to be small and round, even if flat and coarse with plane mirrors. Thus 

 with concave mirrors the interposed thick plate of glass is not needed. 



PART m. ELLIPTIC INTERFEROMETRY WITH A NERNST FILAMENT. 



61. Introduction. The ideal illumination for the present purposes is sun- 

 light, inasmuch as the lines of the spectrum are always present ; but it is not 

 generally available. In its absence the electric arc does good service. Here 

 the sodium line is always visible and of sufficient intensity to be utilized as a 

 landmark, if desirable. It is preferable to use the arc without condensers 

 and to place it several feet from the slit, in order that the rays may be nearly 

 parallel and that the pencil which comes out of the objective of the collimator 

 may be a nearly vertical sheet, converging toward the opaque mirrors. Such 

 a rapier-like beam is more easily made to penetrate long tubes and similar 

 appliances. If the interferences are to be sharp, the lateral extent of the pencil 

 must be as narrow as practicable. Moreover, while the rays from the colli- 

 mator are parallel in their horizontal projection, they are not so in their 

 vertical projection, for which case the focus may as a rule be advantageously 

 placed (by moving the electric arc to or from the slit) at the opaque mirrors 

 of the interferometer. 



The arc lamp has one great objection, however, inasmuch as the mobile 

 arc requires constant attention, and even in such a case adequate illumination 

 often fails at a critical moment. It is therefore desirable to find a more steady 

 source of illumination of sufficient intensity, and this is clearly attainable with 

 the use of the Nernst burner. Experiments were therefore made with this 

 light and the following compact form of apparatus which satisfies many pur- 



