THE AID OF THE ACHROMATIC FRINGES. 



37 



the transverse arrow in figure 1 9 (indicating the right and left side of the slit- 

 images) show, after the reflections at p m n P, although the slit-images are not 

 reversed, the superposed rays are reversed; for these constitute the right- 

 hand and left-hand radiation from the filament, 

 separated by the prism p. If the slit is widened or 

 removed, there is only one vertical line of rays (co- 

 inciding with the position of the slit before removal) 

 which can interfere. The remaining light does not 

 interfere and its admixture robs the phenomenon 

 proper of its brilliancy. 



A few experiments made on the nature of the achromatic phenomenon here 

 obtained showed that the fringes are probably Fresnellian interferences. To 

 test this the objective of the collimator was removed and strong fringes were 

 obtained by passing the two washed images of the slit over each other laterally, 

 by moving the corresponding adjustment screw on the mirror n. It was 

 found that the fringes passed from horizontal maxima in size, gradually to 

 vertical hair-lines, as the images slid horizontally from contact of their nearer 

 edges to the contact of the further edges. The coarse fringes were even 

 strongly present in the narrow gap between slit-images before contact. The 

 telescope was now focussed on the slit, so that sharp linear images appeared. 

 The fringes vanished; but it appeared that the coarse fringes corresponded 

 to coincident sharp slit-images when observed out of focus, and the fine 

 fringes to sharp slit-images far apart. The whole phenomenon thus depends 

 on the distance apart of two lines of light and the interferences are observ- 

 able before or behind their plane. 



20. Reversed rays. The apparatus was now adjusted for the reversal of 

 the rays d' by adjusting a mirror at some place n" (fig. 15) on a fixed microm- 

 eter and in such a way that the rays on reflection retraced their path. The 

 mirror at n being a half-silvered plate, in turn reflected the 

 rays toward the prism P. This modification of apparatus in- 

 troduces very considerable path-difference, 2 nn", on the right, 

 which must therefore be compensated on the left. It is diffi- 

 cult to accommodate the micrometer and leveling devices at 

 n and n" without an allowance of 5 to 10 cm. of path-excess. 

 In my first experiments, which were merely tentative, the com- 

 pensation on the left was secured by inserting a glass column 

 about 15 cm. long. With this and the right-and-left microm- 

 eter displacement of the prism, or the to-and-fro motion of 

 the mirror n", path-difference was easily annulled and the 

 ellipses found in the spectrum. They are centered as usual by rotating the 

 glass compensator on a horizontal and a vertical axis, till with the occurrence 

 of parallel rays at T the illuminated strips on the prism coincide, locally, to 

 the eye. 



In view of the long glass path and therefore of considerable dispersive effect, 



