1275 
av + by +cz—d is a plane, we pass to another plane of the same 
kind by putting: 
aw + by dee =d + (a,a + Bb + y,C)a 
where a, 8,7, are whole numbers. Now the distance of the two 
planes considered is 
a 
es (Oy ate ae) 
Ve be 
which, abc being given, must be a minimum. This minimum is 
reached if a@,,8,,y,, are such that 
a, + Bb Hye = 1. 
, 6 and c being given, this equation can always be satisfied in oo? 
ways. The minimum distance of the planes I will represent by ln» We 
may still observe that in applying the above results we have the means of 
easily comparing the number of molecules lying in the different planes. 
The number of molecules that each plane contains will be greater, 
the greater the distance of the planes of a given kind is. If the number 
of molecules pro unit of volume is », then a plane with parameters 
. YP . . 
a be, contains molecules pro unit of surface. 
Wabe 
The plane of the kind considered, denoted by the parameter s, 
contains MN, molecules. The contribution to the vector of radiation, 
originating from this plane, thus amounts to 
NsA (; r 231 bn ) 
608 225) — —— = 
r 1 A Vo? db? HC 
Taking the sum with respect to s over all possible values, then 
we obtain the total vector of radiation originating from the emission 
of molecules. Generally, however, the contributions to the vector of 
radiation here considered and originating from parallel planes, are 
a bn 
Var bre 
are mutually measurable. If we have to do with several wave- 
lengths, this will certainly cause incoherence. 
Now, the intensity of the maxima observed can AE be found 
if for a moment we imagine an equal number of points getting into 
vibration in all planes considered. Then, if » is the number of planes 
considered, the intensity is 
incoherent, unless, which may exceptionally occur, À and 
nN’, 
where nN? is therefore substituted for 
NG? 
