OF NITRO-CELLULOSE AND THE LAW OF ITS OPTICAL BEHAVIOUR. 
159 
Table III. — First Moments of Stress-strain Curves = M'. 
x h 
A. 
W = 20 lb. 
18 lb. 
16 lb. 
14 lb. 
12 lb. 
10 lb. 
9 lb. 
M = 198 
lb./in. 
178-3 
lb./in. 
158-7 
lb./in. 
139-2 
lb./in. 
119-6 
lb./in. 
100-0 
lb./in. 
90-2 
lb./in. 
20 
4725 
46-20 
40-30 
33-50 
29-65 
25-60 
25 
4925 
43-15 
38-40 
32-15 
28 • 65 
24-35 
— 
— 
30 
5135 
41 • 25 
36-50 
31-80 
27-00 
23-35 
19-95 
17-90 
35 
5380 
39-85 
36-40 
31-10 
25 • 75 
21-20 
18-25 
— 
40 
5660 
38-00 
34-65 
28-65 
24-00 
20-55 
17-50 
— 
45 
6015 
36 • 65 
31-90 
27-85 
23-00 
19-25 
17-00 
— 
50 
6430 
33-50 
29-55 
25-00 
21-15 
18-60 
15-35 
— 
retardation is assumed to be independent of the wave-length, the mean value of 
affords values of /3 corresponding to different wave-lengths. The values of /3 
determined in this way are shown in the accompanying Table IV. 
Table IV. 
*i. 
A. 
P. 
20 
4725 
920 
25 
4925 
960 
30 
5135 
1000 
35 
5380 
1050 
40 
5660 
1104 
45 
6015 
1172 
50 
6430 
1255 
In order to determine the scale of strains the value of Young’s modulus 
E = 309,000, as taken from the measurements in tension within the elastic limit, 
is assumed to hold near the neutral axis of the beam for all loads, and since in this 
region we have the strain e = Y. s, where s is the scale for strains, then 
^ df ($ dX 
E = * . 5?’ 
or 
/3 dX 
S ~E'dY' 
dX 
The slopes ^ near the neutral axis are measured from the diagrams similar 
to those of fig. 15, and their values are shown in the accompanying Table V. 
