153 
res^ectmg the Injlcxion of Light. 
to the Interference of the direct rays with rays reflected from the 
margin of the inflecting body. M, Fresnel, however, has since 
found, that this explanation is insufficient ; for the real place of 
the fringe is sometimes jV^th of a millimetre different from what 
it should be upon this supposition ; and, at any rate, if the hy- 
pothesis were true, the extent and curvature of the margin of the 
inflecting body would have an influence upon the intensity of 
the fringes, which is contrary to experiment. He is, therefore, 
obliged to admit, that the rays which pass at a sensible distance 
from the inflecting body, are made to deviate from their primi- 
tive direction, and concur also in the production of the coloured 
fringes. 
It is by no means easy to explain to general readers the hy- 
pothesis by which M. Fresnel has accounted for the fringes up- 
on the principle of interference ; but we shall attempt to make 
it as intelligible as possible. 
Let AME, Plate IV. Fig. 5. be a luminous wave or undula- 
tion, propagated from the radiant point C, and partly intercep- 
ted by the inflecting body AG. Then, if we suppose it divided 
into an infinity of small arcs Aw, w'w, wM, Mw, nn', ri'n" &c., 
M. Fresnel obtains the intensity of any point (P) of the wave 
AME, when it has reached the position BPD, by supposing 
elementary waves to be propagated from every point w, w', M, n, 
n\ he. of its preceding position at AME. These elementary 
waves are propagated in all directions, and with intensities sen- 
sibly equal when they do not deviate much from the perpendi- 
cular. M. Fresnel does not take into account the waves which 
are much inclined, and which, according to his hypothesis, de- 
stroy one another ; and in this way he determines the intensity 
of the light resulting from the reciprocal influence of all the rays 
which are slightly inclined to the perpendicular. By thus com- 
bining the principle of Huygens with Dr Young’s law of inter- 
ference, he has obtained a formula which represents the obser- 
vations with surprising accuracy. 
7. The phenomena of inflexion are considered by M. Fresnel 
to be inexplicable on the Newtonian theory of the emission of lu- 
minous particles ; while almost all of them may be directly de- 
duced from the Huygenian Theory of Undulations 
* Such of our readers as are desirous of farther information respecting M. Fresnel’s, 
discoveries, are referred to the Ann. de Chim. et de Phys. tom. xi. p. 246 & 33T, 
