PHYSICS: C. BARUS 
613 
The fringes in the zeroth order were good and strong not inferior to 
any of the others, but unfortunately too shortlived. In the fourth 
order the fringes are weak (although the enormous sodium doublets 
stand out clearly), doubtless from excess of extraneous light. Here 
also it is difficult to prevent the beam from vanishing at the edge of the 
prism P^ Hence the anomalously small displacement, a discrepancy 
already quite manifest in the third order. 
The present experiments furnish a striking example of the uniform 
breadth of the strip of spectrum carrying the fringes, quite apart from 
the dispersion of the spectra. In the prism spectrum, where the sodium 
doublets are indicated by a hair line just visible, to the fourth order 
spectra, where they stand apart like ropes, the linear interference 
phenomenon has the same width. 
The computation of the dispersive power in these cases is peculiar. 
It will be seen from figure 3, that the angle (8 = 44°) between the 
incident ray a and the diffracted ray b is constant and is 5 = ^ + i in 
the first and second and 8 = 6 — tin the third and fourth order. Hence 
in succession (i changing sign after the 2d order) 
sin (8 — i) — sini = \/D, 
sin {8 — i) — sin i = 2\/D, etc., 
from which equations the angle i may be computed. I did this with 
sufficient accuracy graphically. 
Since dd = di, apart from sign, it follows that the dispersing power 
is 
-dd/d\ = n/D (cos i + cos {8 - i) ) 
where n is the order of the spectrum. With the values of i given, the 
data for d6/d\ were finally computed. The dispersive power of the 
prism was computed as above and is to be added to all the succeeding 
dispersive powers. The path difference, x = 2e cos 8/2 here corre- 
sponds to 2y above. Through the second order the rate x/{dd/d\) is 
about 120 X 10~^. This value is larger than the former owing to inci- 
dental conditions. Similarly the proportionate effect of dispersion 
breaks down in the third and fourth orders, as already stated. The 
spectra themselves were still adequately bright, but the fringes were 
faint for some reason and I failed to make them stronger. 
