ELECTRIC RADIATIONS—BRAGG. 203 
Lenard found that his results could be accounted for on the suppo- 
sition that there was an absorption according to an exponential law, 
over and above the weakening due to spreading from a center. 
If a 8 particle or cathode particle were lable to complete absorp- 
tion by an atom which it entered, such an exponential law would 
result at once. As a matter of fact, it looks as if several violent de- 
flections might take place before the final disappearance of the par- 
ticle’s activity. It looks, also, I think, as if deflections were usually 
not at all great during the progress of the particle through the atom, 
but were apt to be severe when they did happen, as if, in fact, the 
field of force which deflected the particle was strong but circum- 
scribed. This would happen if the positives and negatives were 
arranged in doublets. When a particle is deflected from a beam 
crossing a thin plate, it starts off on a new path which leads much 
less directly to the open air, and its velocity is somewhat diminished. 
It may be, therefore, that the infrequency but severity of the parti- 
cle’s encounters makes it possible to look upon each encounter as an 
absolute, or at least a definite, loss to the stream, so that an expo- 
nential law results. 
Certainly the application of this law to the interpretation of ex- 
periments has had very great success, both in respect to cathode 
and to 8 and y rays. As examples of the latter we may take Ruther- 
ford’s determination of the absorption of the 8 rays of uranium and 
Godlewski’s similar determination for actinium. (Jahrbuch der 
Rad. und Elek., Bd. IIT, Heft 2, p. 159.) In experiments of this 
kind the radiating material is spread evenly on a level surface, and 
sheets of absorbing material are placed upon it. The ionization 
produced in the space above the sheets is compared with the thick- 
ness of the sheets, and the two variables are found to be connected 
together more or less exactly by an exponential law. There is some 
difficulty in determining whether such measurements give more 
nearly the number or the energy of the stream of particles which 
emerges from the plate, as Rutherford (“ Radioactivity,” 2d ed., p. 
134) and Thomson (“ Conduction through Gases,” 2d ed., p. 375) 
have pointed out. The point was also discussed in my address to 
Section A of the Australasian Association for the Advancement of 
Science, Dunedin, 1904, page 69. There is also an uncertainty due to 
the application of a formula to radiation from an assemblage of 
points which is really only applicable to a plane wave, or a stream 
moving normally to the plate. If a point source of radiation is 
placed below an absorbing plate of thickness d, and there is a true 
coefficient of absorption A, the fraction that emerges from the fur- 
ther side of the plate is not e-Ad; much of the radiation passes 
obliquely through the plate and is absorbed to a greater degree 
