240 
PHYSICS: F. C. BLAKE 
Now in the work of Blake and Duane 5 was 0.040 cm. Taking ju 
= 4°18', D' comes out 0.150 mm.; and D" , 0.363 mm. 
If we plot the relative intensity, viz. 
4.94 and 
I I — e 2 sind cos d ) 
against x, we get Curve I of figure 2 for values of x less than 0.04 cm. 
values of x greater than 0.04 cm. the relative intensity is 
For 
' _ \x (x+s) 
g 2 sin 0 cos d — £ sin $ cos { 
■): 
Value of X tn 
/oo 
FIG. 2. 
and plotting this against x, we get Curve II of figure 2. Measured carefully 
with a planimeter the average ordinate for I is 2.41 and for II it is 0.76. Finally 
therefore we have as the depth of the effective plane 
D 
2.41 ZT + 0.76 D" 
3.17 
= 0.201 mm. 
Blake and Duane found D experimentally to be 0.203 mm. Thus the agree- 
ment is all that can be expected. 
In figure 1, for simplicity sake the writer has assumed the space lattice for 
calcite to be cubical; as a matter of fact it is rhombohedral. Blake and Duane 
