Dr. Young's Lecture on 
26 
to its strength by a divergence of motion from any other parts 
of the undulation, for want of a coincidence in time, as has 
already been explained with respect to the various force of a 
spherical undulation. If indeed the aperture bear but a small 
proportion to the breadth of an undulation, the newly generated 
undulation may nearly absorb the whole force of the portion 
admitted ; and this is the case considered by Newton in the 
Principia. But no experiment can be made under these circum- 
stances with light, on account of the minuteness of its undula- 
tions, and the interference of inflection; and yet some faint 
radiations do actually diverge beyond any probable limits of 
inflection, rendering the margin of the aperture distinctly visible 
in all directions ; these are attributed by Newton to some un- 
known cause, distinct from inflection ; ( Optics, Third Book, 
Obs. 5.) and they fully answer the description of this propo- 
sition. 
Let the concentric lines in Fig. 1. (Plate I.) represent the con- 
temporaneous situation of similar parts of a number of suc- 
cessive undulations diverging from the point A ; they will also 
represent the successive situations of each individual undulation: 
let the force of each undulation be represented by the breadth of 
the line, and let the cone of light ABC be admitted through 
the aperture BC ; then the principal undulations will proceed 
in a rectilinear direction towards GH, and the faint radiations 
on each side will diverge from B and C as centres, without 
receiving any additional force from any intermediate point D 
of the undulation, on account of the inequality of the lines DE 
and DF. But, if we allow some little lateral divergence from 
the extremities of the undulations, it must diminish their force, 
without adding materially to that of the dissipated light; and their 
