612 
PHYSICS: C. BARUS 
(sunlight should be focused by a large lens of long focus — 5 feet — on the 
slit). The rays a, a' , are then either reflected or diffracted in any order, 
by the identical plane reflecting gratings G, G\ into the collinear rays 
b, b\ These are reflected by the silvered right angled prism P' and 
observed in a telescope at T. G and and also should be on mi- 
crometers, so that the corresponding displacements, e, e\ normal to G 
and G' and y in the direction b b' , may be registered. 
The adjustments, if symmetry were demanded, would be cumber- 
some; for in addition to precise modification of the position and orienta- 
tion of the prisms P, P\ the grating requires fine adjustment and a 
means of securing parallelism of the rulings. But an approximate 
adjustment does very well and no pains were taken to secure symmetry. 
The spectra were brilliant in the low order work; but even in the fourth 
order the light was adequate. One may note that the gratings enhance 
the dispersion of the prism P, which is relatively small. Table 2 is an 
example of results. 
TABLE 2 
Ranges or Displacement, e, Varying with Dispersion. Paired Gratings CSpace 
= 200 X 10-« cm) AND 60° Prism. 0 = 46°, 5 = 44°. Path Difference x = 2e cos 5/2 
OBSERVED 
ORDER 
eX 103 
xX 103 
dd/d\ 
cm. 
cm. 
0 
38 
70 
760 
1 
190 
350 
3,490 
2 
420 
780 
6,440 
31 
520 
962 
9,930 
41 
580 
1,070 
14,800 
Fringes too faint. 
