PHYSICS: C. BARUS 611 
and ending with b. The faint fringes follow the rules of displacement 
interferometry. 
TABLE 1 
Range of Displacement, e, y, Varying with Dispersion 
METHOD 
yX 103 
d 
dd/d\ 
cm. 
cm. 
cm. 
60** prism at P 
28 
24 
21 
49^45' 
760 
Grating {D - 352 X 10-«) at P 
161 
136 
108 
9039/ 
2,880 
250 
230 
155 
19''34' 
6,030 
Film grating (D = 167 X IQ-s) at P 
302 
236 
190 
20°40' 
6,400 
Same. 2d order 
470 
420 
440 
44°56' 
16,900 
In addition to the data of the table, a large number of miscellaneous 
tests were made with the reflecting prism in different positions. Unless 
brought too far to the rear, when the beams are lost at the edge and e 
too small, the results for fine and coarser fringes were of the same order. 
The data for e are not sufficiently regular in the dispersive powers 
above 1000 for graphic treatment (it is probable that at 16,900 the 
sliding along the prism surface is interfered with) but the data for the 
path difference, 2y, are available. All the data, in consideration of 
their limitations, bear out the inference that the range of displacement 
within which fringes are seen, increases in marked degree with the 
dispersion, the average initial ratio 2y/{d9/d\) is about 60 X 10~^ cm. 
A very surprising result in these experiments is the efficiency of the 
film grating in series IV and V, not only in the first but in the second 
order of spectra. 
After these experiments an attempt was made to obtain similar results 
with the more comprehensive method of two gratings G and (trans- 
mitting G at P and reflecting at P') with an appropriate change of 
the angle 8. But here the choice of gratings with satisfactory con- 
stants was limited and with high double dispersion the fields were apt 
to be too dark. Good results were obtained with the 60° prism and 
concave grating and with the ruled grating together with the latter. 
The data again showed marked increase of displacement with the dis- 
persion dd/d\. 
2. Diffraction at M, N, replacing reflection. — The present method of 
observing interferences in the zeroth, first, second, third and even 
fourth order, successively, without essential change of the parts of the 
apparatus, is noteworthy. In figure 3, the incident light L from the 
collimator is separated into two component beams a and a' by the 60° 
prism, P. This is essential here, as an abundance of light is needed 
