82 



THE INTERFEROMETRY OF 



and the independent and separated component beams Mm and Nn are con- 

 veniently accessible. 



The experiments with homogeneous light (sodium arc) gave perfectly 

 regular striations covering the whole of the wide slit image, uniformly. With 

 glass compensators 0.6 to o.i cm. or more thick on both sides, the striations 

 became somewhat smaller, as was to be anticipated. Fringes could be erected 

 and enlarged by rotating the grating on an axis normal to its face and by 

 other corresponding rotations. The fringes, as a whole, were large and 

 splendid and suitable for general purposes in interferometry. 



37. Experiments. Transmitting grating. Crossed rays. The second posi- 

 tion of this apparatus was now tested, the rays passing along the diagonal 

 of the rectangle (fig. 59) and crossing at G in the grating. The interfering 

 pencils were thus GNGmG and GMGnG. The slit 

 should be quite wide. Seen in the telescope at T, 

 therefore, the dispersion is reduced in virtue of 

 double diffraction, the tendency being toward white 

 slit images, as already explained. A variety of very 

 interesting results were obtained after the interfer- 

 ences had been found. The outgoing and returning 

 paths are coincident, and both component rays pass 

 through the grating two times, the ruled face being 

 towards the telescope. 



The adjustment is at first somewhat difficult. 

 Having made a rough setting of the mirrors as to 

 distance, etc., by the aid of sunlight or arc light, so 

 that the spectra may be seen, two wide slit images 

 will appear in the telescope T, but they will usually 

 be differently colored. The mirrors m and n are then to be rotated around 

 vertical axes (fine-screw motion) until both slit images are identically colored 

 and coincide. After this, homogeneous light (sodium arc) must be used and 

 the rotation of mirrors on the vertical and horizontal axes repeated until both 

 fields are identically yellow on coincidence. The sharply focussed edges of 

 the wide slit are now the vertical and horizontal guide-lines for adjustment. 

 All corresponding lines must coincide if the phenomenon is to be obtainable. 

 Thereafter the micrometer at M, actuating the mirror fore-and-aft parallel 

 to itself, is manipulated till the fringes appear. 



Two types of interference may be observed. The first are variations of 

 nearly equidistant fringe patterns, obtained with homogeneous light only 

 and covering the whole wide slit image on good adjustment. They would 

 appear equally well in the absence of the slit. The second type is obtained 

 in the presence of white light, or of the mixture of white light with the homo- 

 geneous light. It is a linear phenomenon, identical in appearance with the one 

 described in Chapter I, though occurring here in the case of a wide slit. Both 

 are very vivid, and the latter particularly, when at its best, in violent tremor. 



