1300 
P, &, the given primary wave, £, the secondary wavé as a 
consequence of the deviations €, caused by P. Then &, can be 
found from: 
dE 0g, 0 
Ey ya + ak, =. a 
Taking this into account, &, which should be independent of P 
and é, has to satisfy 
EP 
den 
Pope ne + ak, a. aoe 
Here the last’ term may be neglected, since it becomes propor- 
tional to the square of the amplitude of &, and this may be - 
supposed to be very small. For & we further find 
dE 07g OE OE «AE, DAE 
Okan a + aE, & to) et AAN 
It appears at once from this equation that the amplitude of the 
secondary wave &, which has to be proportional to the disturbances 
§, and to the amplitude of §,, is also proportional to « and that 
this wave therefore does not occur if HookE'’s law holds. 
In this case the former method still yielded the unimportant solution 
(6). This solution now does not appear at all, because here 
a different rv-coördinate is used. The 2 used in the former treatment 
is evidently equal to what is represented here by «-+-&,. For the 
rest the equation (10) as regards the form is equal to the equation 
from which, using the other method, the secondary wave is found, 
For the case which we considered the integration furnishes exactly 
the result likewise expressed in (5) and (8). 
4. The problem of the scattering of elastic waves by accidental 
deviations of density ‘is compared by DeBije to the scattering of 
light by those deviations. A great difference of course is that 
light has such a velocity that the molecular velocities, consequently 
also the velocity with which the deviations change, may be neglec- 
ted. Therefore in the optical problem the deviations of density are 
rightly admitted to be at rest. If this is likewise done in the 
case of the analogous elastic problem, nothing but a qualitative 
conformity with reality will be expected. 
In order to demonstrate this unnatural “keeping constant” of the 
deviations, we have spoken in point 3 about statical deviations 
caused by constant forces. Now let us imagine those forces to be 
removed suddenly. 
Then the deviation between o and / gives origin to two disturb- 
ances of equilibrium moving with a velocity g, one to the left, the 
