12 THE INTERFEROMETRY OF 



erecting the interference design C, figure 2. It then took the form given at D, 

 and this seems to furnish the final clue to the subject. In other words, the 

 design consists of a new type of extremely eccentric ellipses, with their long 

 axes parallel to the Fraunhofer lines, each end having the outline of a needle- 

 point, possibly even concave outward. Only one end of the long, closed 

 curves is obtainable. These jet-black lines dance on the highly colored back- 

 ground of less than half the width between the two sodium lines. The inter- 

 ference design would, therefore, be the same (apart from color) as that which 

 would be obtained if the spectrum containing ordinary elliptic interferences 

 were to shrink longitudinally from red to violet, till it occupied less than half 

 the space between the two D lines. In fact, I have at other times obtained 

 just such patterns, with all the colors present, but not in the pure yellow, as in 

 the present case. Vertically, the path-difference is always due to more or less 

 obliquity of the rays passing through the plate of the grating. Horizontally, 

 however, the equivalent path-difference is complicated, in the present case, by 

 the fact that one wave-length of a pair has increased, whereas the other has 

 diminished, while both may pass through the same thickness of glass and air. 



4. Coincident spectra with one reversed on a given longitudinal axis. For 



this experiment it is necessary to reflect the first-order spectra issuing at 

 the grating G, figure 4, from the ruled face g (a narrow, preferably horizontal, 

 blade of white light is here furnished by the collimator L with a horizontal 

 slit, and the rulings of the grating are also horizontal and parallel to it), twice 

 in succession and preferably from mirrors M and N and in and n, reflecting 

 normally to each other and inclined at an angle of, roughly, 45. Each of 

 the mirrors M and N must be revolvable about a horizontal axis parallel to 

 the slit and furnished with three adjustment screws relatively to axes normal 

 to each other, one of which is horizontal. The mirrors m, n are the silvered 

 faces of a prism right-angled at the edge. It is, moreover, to be placed on 

 the slide of a Fraunhofer micrometer so that the prism may be moved, grad- 

 ually up and down, for the adjustment of distances. 



On leaving the mirror m, n, the two spectra are carried by nearly horizontal 

 and parallel sheets of divergent rays, which pass outward from the diagram. 

 But it will be seen that one of the two spectra reaching the observer is reversed 

 on the longitudinal axis relatively to the other; i.e., if one is in the position 



, f top 1 , ( bottom ) . , 



red ] , I violet, the other will be red ] > violet. 



( bottom j ( top J 



The subsequent passage of the rays is shown in figure 5, which is the side 

 elevation and therefore at right angles to the preceding figure. The rays 

 from m and n impinge on a distant, slightly concave mirror K (about 1.74 

 meters in focal distance) , placed somewhat obliquely, so that when the rays 

 come to a focus at F near the micrometer they may just avoid it. The partially 

 overlapping spectra at F are viewed by a strong eyepiece, E. The observer 

 at E can then control the Fraunhofer micrometer by which m, n is raised and 

 lowered, and the three adjustment screws of M. 



