REVERSED AND NON-REVERSED SPECTRA. 71 



whereas on pushing the ocular in from the principal position the separation 

 is increased. Hence, to account for the disturbance in the ether gap, as it 

 were, it seems most reasonable to assume that the rays cross and interference 

 occurs after the rays have passed the principal focal plane (i.e., nearer the 

 eye of the observer), and that the interferences occurring here are projected 

 into the principal focal plane. Nevertheless, the fringes are so strong and 

 sharp that the two clearly focused spectra seem, to react on each other across 

 the gap at their edges. I have pointed out similar phenomena in the pre- 

 ceding report (Carnegie Inst. Wash. Pub. 249). The case is just as if a tele- 

 scope or lens focused on a single or a Young's double slit (with the images 

 sharply delineated) should show fringes. 



In adjusting the interferometer, figure 43, for these experiments, the fol- 

 lowing systematic plan was pursued. By a rough adjustment with sunlight 

 and measurement, all parts of the apparatus are first placed symmetrically 

 to each other (as in figure). The direct beam should just graze the edge of 

 the prism P' and the naked eye, viewing the edge from above, should see the 

 two bright rays of the same color (reflected from M and N) contiguously 

 near the edge. In the spectra of the same length on the two sides of the 

 prism P', the same colors are opposite each other. On looking down on the 

 edge of the prism with a telescope (fine slit), two sharp and clear spectra 

 should be seen, which can be made to overlap at their edges in any amount 

 by rotating M and AT on a horizontal axis. Finally, the D lines are brought 

 into coincidence by rotating the grating G on a vertical axis. The largest 

 fringes are obtained by slightly raising and lowering the incident beam L to 

 the grazing position in question. By displacing P on the micrometer, right 

 and left, the fringes are soon found. 



The first experiments were made with the object of testing the effect of a 

 slant, to the right or left, of the edge of the prism on the range of displace- 

 ment y. With a 60 prism at P, the search was found to be too difficult and 

 therefore soon was given up. The few data obtained were: Edge of P' sym- 

 metrical range, ^=0.06 1, 0.050, 0.062 cm.; edge of P' toward left range, 

 ^=0.63 cm.; showing no certain difference. 



P was therefore replaced by a ruled grating (D = 3 52X10-* cm.) to obtain 

 greater dispersion. The ranges of displacement, y, now found were: edge of 

 P' to left, y =0.120, 0.130 cm.; edge of P' symmetrical, ^=0.128, 0.127 cm.; 

 edge of P' to right ^ = 0.110, 0.113 cm.; readjusted, 0.130, 0.132 cm. These 

 differences are apparently incidental, as much depends on the light. In a 

 darker field fringes vanish sooner. One may assume that slight inclinations 

 of the edge of the reflecting prism P' are without consequence. 



The next experiment was to determine the effect of a lack of collinearity 

 of the rays b b'. This shows itself to the naked eye looking down upon the 

 edge of the prism from above, since by rotating M and N in contrary direc- 

 tions around a vertical axis the bright spots of light move along the edge of 

 the prism from front to rear, or the reverse. If regarded by a telescope, 

 the axis of the instrument will be correspondingly inclined toward the front 



