98 THE INTERFEROMETRY OF 



Gz, G'i: Distance 12 cm. Coarse irregular; with lens fine regular 



set, near and beyond the surfaces. 

 G2, G'i: Distance 45 cm. Surfaces with doubled fine striations; 



with lens finally strong and regular. 

 G2, G'i: Distance 60 cm. Regular faint; irregular double, very 



strong; surfaces striated; with lens strong double irregular; 



finally regular small. 

 3, G'I: Regular; regular line split; irregular coarse; surfaces finely 



striated, G coarser; fringes grow continually larger without 



vanishing. 



On moving G fore and aft, two grids seem to travel through each other in 

 opposite directions. This probably accounts for the occurrence of irregular 

 fringes. The size of fringes seems to be a minimum for a conjugate focus 

 near the surfaces. The whole phenomenon is continuous. Irregular fringes, 

 probably superpositions, become regular in other focal planes. 



3 , G f 2 : About the same ; minimum size at the surfaces, increasing 

 about three times as the ocular is drawn either way. 



6*3, G'Q\ also 6*3, mirror: About the same results, only brighter and 

 better. Hence in case of large dispersion two gratings are not 

 needed. The two sodium lines, when the ocular is drawn out 

 of focus, multiply themselves at regular intervals, so that the 

 grids are sometimes distinct, sometimes partially superposed. 

 Thus the classic diffraction phenomena of a slit suggest them- 

 selves as the starting-point for an explanation of the present 

 phenomena as a whole. 



3, G'o, produced alternately with sodium light and sunlight, showed 

 the same sequence of fringes (the large ones with a tendency 

 to split) in the former case, while nothing appeared in the case 

 of white light. 



49. Grating on a spectrometer. It seemed necessary, therefore, to con- 

 sider the diffraction of a fine slit, when seen in the telescope, somewhat in 

 detail. In Chapter III the production of beautiful Fresnellian interferences 

 from two identical slit images and homogeneous light was demonstrated; but 

 an equally clear manifestation of the diffraction of a slit image, when the 

 ocular is out of focus, does not seem to occur. The broad image of the slit 

 out of focus shows a stringy structure only, but no separation is easily obtain- 

 able. Fringes, as such, are quite absent when the ocular is drawn out. 



The light of the sodium arc was now passed through a very fine slit and 

 collimator and reflected from a plate grating. The above intermittently 

 regular and irregular fringes were strikingly obtained with the ocular out of 

 focus. As this is successively more and more drawn out, fine lines become 

 coarser, and then seem to subdivide, giving the structure a fluted appear- 

 ance, frequently regular. There is, in other words, a double periodicity. In 



