364 
PHYSICS: C. BARUS 
following data for the micrometer position corresponding to the clearest 
demarcations of yellow fringes are illustrative. At least six periods 
{n) are easily detected in each side of the ovals {n = 0) Thus, 
n 6 5 4 3 2 1 0 etc. 
AiV^ X 103 0 49 90 127 171 214 243 cm. etc. 
These intervals, since it is impossible to establish the maximums states 
of presence or absence of fringes quite sharply, are practically equi- 
distant as the similar series in figure 4 indi- 
cates. Fringes in presence are here shown by 
black dots, the circles denote clear fields. 
Thus the mean period of reappearance is 
A AT" = 0.042 cm.; or a path difference of 
2 A iV cos 0 = 0.059 cm; or a shift of ray 
parallel to itself (2 A sin (9 = 0.059 cm.) 
of the same amount. 
The reason for this rhythm can only be the two wave lengths of the 
Di and D2 lines of the sodium flame, originally detected in the colors 
of thin plates by Fizeau. Hence a relatively enormous shift of microm- 
eter of nearly half a millimeter is equivalent to the wave length in- 
terval A X = 6 X 10-8 cm.; or A X / A iV = 6 X lO-^ / 42 X IQ-^ = 
1.4 X 10"^ Treating the case in terms of the interferences of thin 
plates and two wavelengths X and X + J X 
n\ = constant, or — = — = — for each period. 
\ n n 
Flence n\=— = 2AN cos d or AA^ = XV2AX cos B 
AX 
since {d = 45°) approximately X = 60 X 10"^ cm. A X = 6 X 10"^ cm. 
cos d = 0.71 A N = 0.041 cm., 
agreeing as nearly as may be expected with the experimental datum. 
The apparatus thus serves incidentally for investigating such properties 
of spectrum lines as Michelson in particular has detected. 
With white light the interference grid rarely reappears rhythmically 
nor does it correspond to the zero'th period of figure 4, i.e., to the ovals 
for sodium light. It is usually exactly of the size of some higher order, 
in yellow light, as at ^^wh^' in fig. 4. It does not require such sharp 
horizontal and vertical coincidence of the superposed images, as the 
spectrum fringes. 
In a flash of the arc, showing many sharp spectrum lines in all colors. 
4- 
4 
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yM 
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