6!0 
PHYSICS: C. BARUS 
being used throughout. Later, the grating method is to be suitably 
modified for corroborative experiments. 
The first series of measurements was obtained with a 60° prism at 
F, the dispersive power dd/d\ being computed (approximately) from 
Cauchy's equation, so that in wave length X 
d8/d\ = 4:B sin J <p/\^ cos i ((p + 8), 
<p being the prism angle (60°) and 5 the angle of minimum deviation. 
The dispersion constant B was estimated to be 4.6 X 10"' 
In the remaining series with a grating at P, dd/d\ = 1/D cos 6, the 
usual expression, 6 being the angle of diffraction and D the grating 
space. The dispersive power thus increases from about 800 to 17,000, 
over twenty times. Throughout this whole enormous range good 
fringes were obtained. 
The values, e, show the normal displacements of the opaque mirror 
My during the presence of fringes, and of the opaque mirror iV, as speci- 
fied. Of these, is systematically larger than for reasons due to 
residual curvature in the mirrors and surfaces, whereby fringes on the 
left (N) vanish sooner than those on the right (M). The datum, y, is 
the displacement of the right angled reflecting prism P', parallel to the 
component rays bb\ This value is necessarily smaller than e, as will 
be shown elsewhere. All measurements were frequently repeated and 
the means finally taken for comparison with dd/d\. 
In the experiments with a ruled grating at P and a concave grating 
reflecting at P', the phenomenon of figure 2 was observed. A wide 
field of faint fringes was visible, enormously accentuated and clear 
in the narrow strip of the linear phenomenon. As the micrometer 
mirror at M moves forward, these faint fringes shift bodily across the 
stationary bright linear strip, beginning therefore with the pattern a 
