204 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1907. 
than that which passes normally. This has often been pointed out, 
e. g., by N. R. Campbell (Phil. Mag., April, 1905, p. 541), who also 
gives some figures from which the proper curve of absorption may 
be drawn. I am not aware, however, that it has been noticed that 
the form of the absorption curve, which is far from an exponential 
curve for a thin radiating layer, approximates much more closely 
to it for a thick radiating layer. And it is interesting to find that 
the experimental curves which are most nearly exponential are those 
for which the layers of radioactive material were thick compared 
to the penetration of the rays under investigation. As examples, 
we may take those of uranium and actinium already mentioned. 
On the other hand, the curve which H. W. Schmidt (Ann. d. Phys., 
Bd. XXI, 1906, p. 651) has obtained for the 6 rays of RaC, the 
radioactive material being deposited in a very thin layer on metal 
foil, shows just about the amount of departure from the exponential 
form which is to be expected if the absorption is truly exponential, 
and there is only one absorption coefficient, not two, as Schmidt has 
suggested. 
The following figures give the proportional amount of the original 
radiation which passes through a plate of thickness 7/A, where A is 
the absorption coefficient: (1) for a thin layer; (2) for a thick layer. 
The figures are also given, for the sake of comparison, for the case 
of a plane wave, or a pencil of rays passing through the plate 
normally. 
Plane 
Radiation | Radiation wave 
n. from thin | from thick (purely 
layer. layer. ex ponen- 
tial). 
(— 1.000 1.000 1.000 
Be ee 723 834 905 
py epee -573 - 102 .819 
poe ee -467 -600 - 742 
Fy ere 387 -510 .671 
5s Sa oe 323 437 -607 
(62525. 2 274 .378 -548 
A er -235 328 -498 
Osea ee -200 283 450 
i eee 171 -248 -405 
1) Qe 2145 214 .368 
The absorption of a material used in a thin sheet naturally appears 
greater than the absorption when the thickness of material is in- 
creased, because the rays which are moving obliquely are absorbed 
first. 
The absorption of y and X rays appears to follow a purely expo- 
nential law so far as experiment has been made. The 8 rays are 
absorbed by molecules immediately on their production. 
