46 DISPLACEMENT INTERFEROMETRY BY 



These results agree much better with the theoretical equation than the 

 former, and may be considered as coinciding with it. 



In the present case the attempt to get interference from rough surfaces 

 was not at first successful. The slit-images are reversed, as indicated by 

 the transverse arrows in figure 2 2 . Hence if the white slit-images are wide 

 there can be coincidence only in a single vertical line. Fringes with white 

 light will occur as a case of the interference of fine slit-images. To produce 

 them it is first necessary to obtain the spectrum fringes with the ellipses, 

 or else with horizontal fringes in the field. If now the spectroscope is removed 

 and the white slit-images put out of focus, the phenomenon indicated in 

 figure 24, where 5 is the superposed, washed slit-images wiJl usually appear, or 

 may be found on cautiously moving the micrometer screw. Within the slit- 

 image the fringes are coarse and colored, but they send out fine oblique stream- 

 ers into the field of diffuse light or glare, on both sides of s. 

 When the slit is widened these fringes are liable to vanish just 24 j//// -> 

 as the spectrum fringes vanish, except perhaps at the edges 

 of the images. These achromatic fringes climb up and down 

 the slit-image with motion of the micrometers (A/V, Aa) with 

 extreme rapidity and are easily lost, as there are not usually 

 more than 10 or 20 of them. If the spectrum ellipses are huge, the white 

 fringes are almost too coarse to be seen and too mobile to be controlled. 



I next removed the objective of the collimator. The fringes, though much 

 changed in appearance, practically black and white, were not destroyed. 

 In such a case the slit-image shrinks vertically. To obtain a long strip a 

 highly illuminated ground-glass screen (sunlight and weak condenser) 

 should be placed in front of the slit as a source of very diffuse light. In 

 such a case this long white post (as it were) is covered from top to bottom 

 with sharp blackish and usually oblique lines, which vanish at once, up 

 or down, on moving the micrometer. No fringes are seen if the slit is in 

 focus. When considerably out of focus (as in case of the diffraction patch 

 in fig. 25,) strong, sharp-colored cross-markings are present, which would 

 be quite available for measurement. However, in this experiment, when the 

 slit was widened or removed, the fringes apparently vanished. The phe- 

 nomena as a whole seem to me to be fringes of the two white slit-images, 

 and seen either behind or in front of their focal plane, like the complemen- 

 tary fringes described elsewhere. This is confirmed by experiments pres- 

 ently to be described. 



24. Reversed rays. The apparatus was now adjusted for a reversal of 

 rays by putting a half -silver plate at n and an opaque mirror on a micrometer 

 (with the screw normal to its face) at some position n" (fig. 23) fixed inde- 

 pendently of the rotation. In this case, therefore, the intercept nn' changes 

 sign. Moreover, the angle of incidence at n" is o. Hence the equation 

 should be 



a = b (sin sin 7) 



