206 PRESNEL. 
is not every inequality of this kind which will necessa- 
rily produce a destruction of light; such difference may, 
on the contrary cause the rays to reinfore each other. 
But when we know what is the least difference of 
route gone through, at which the rays may be super- 
posed without influencing each other, we then obtain all 
the other differences of route which give the same result 
in a very simple manner ; for it suffices to take the 
double, the triple, the quadruple, &c., i. e. every whole 
multiple, of the first number to give them. 
If we have noted in like manner the least diffebiedina 
of route which produces complete destruction of the two 
rays, every odd whole multiple of this first number will 
also be the indication of a like destruction. 
As for differences of route which are not numerically 
comprised either in the first or in the second of the 
above series, they correspond only to partial destruc- 
tions of the light, or mere weakening of its intensity. 
These series of numbers, by aid of which we can tell 
whether two rays at the moment of intersection ought to 
interfere or merely to combine without influencing each 
other, have not the same values for the differently col- 
oured rays ; the smallest values belong to the violet rays, 
the greatest to the red, and the intermediate values to 
the intermediate rays. It results, that if two white rays 
cross at a certain point, it may be possible that in the 
infinite series of differently coloured rays of which that 
light is composed, the red, for example, alone may be 
destroyed and disappear, and thus the point of concourse 
may appear green, as being the white light deprived of 
its red component. 
Interference, then, which in homogeneous light pro- 
duces only changes in intensity, will manifest itself when 
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