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point P of the photographic plate, give at that place conjointly the 
disturbance of equilibrium 
Msin p(t —t4) + Msinp(t—te) .- ve». « » (a) 
The time average of the square of (a4), — taken over a period, is: 
Af 2 
mat + 2oosp (ta —tB) +1] = M [1 + cosp(t4 — tp)] . (B) 
If the two atoms A, B belong to different molecules of the gas, 
the quantity cos p(t1—tzB) assumes equally often equally large positive 
and negative values during the time of exposure in consequence of 
the independent movement of the molecules, so that here the time 
average of cos p(ta—tp) becomes zero. 
It is different with two atoms belonging to the same molecule. 
We split up here the taking of the time average into two phases: 
J. All possible orientations of the axis of the di-atomic molecule, 
one atom being fixed. //. Repetition of this average value deter- 
mination for all possible situations of that atom inside the (irradiated) 
gas space. 
Mean value determination I. 
Let CA be an incident Röntgen ray, „AD the secondary ray that 
travels from atom 1 to the point P of the photographie plate. What 
is the locus of all the situations of the atom B, for which the 
difference of path 
CDA —EBF=BL4 
has one and the same value? ’) Answer: Deseribe round A a sphere 
with radius equal to the fixed distance of the atoms AL= a. 
Intersect this sphere with the plane 5’ YA, which is normal to the 
plane of drawing C’AD, and parallel to the straight line AU”). 
The circle BB’ along which this plane intersects the sphere is the 
required loeus; for all its points: 
BBE = EBS. 
Hence all of them give one and the same difference of path with 
respect to CAD, which can also be represented by 
NEENFZ SEA 
finally also by 
( 
NZ VA We OAT edn LE als 
= 
1) AB is so small compared with AP and BP that AP and BP may be con- 
sidered as parallel, hence A as the difference of path with which the secondary 
waves from A and B arrive in P. 
?) A mirror parallel to the plane Bb’ YL would just reflect the rays Lb’, YX, KB 
in the direction BF’, YZ, BE. 
