Account ofyi. Fresnel’s Discoveries 
formed by masses of the same substance. He examined the ef- 
fects of platinum^ diamond^ and corTe^ in inflecting the light ; the 
effects of grooves in metallic surfaces, &c. ; and of cylinders of 
glass immersed in fluids of the same refractive power : and 
from these he concluded, that the Newtonian theory of inflexion 
could not be true ; that the inflexion was not produced by any 
force inherent in the bodies themselves, but arose from a pro- 
perty of the light itself, which always shewed itself when di- 
vergent light was stopped in its progress 
6. The fringes in the interior of the shadow, were first ex- 
plained by Dr Thomas Young. He shewed in the clearest man- 
ner, that they were formed by the interference of two portions 
of light coming from the opposite sides of the inflecting body. 
Having introduced the sun’s light into a dark room, through 
two small holes very near each other, he received the admitted 
light upon a sheet of paper from each of the holes separately, 
and observed no particular effect. But when the fight was ad- 
mitted through both the holes at the same time, so as to inter- 
fere, a series of obscure and brilliant fringes was produced, 
M. Fresnel obtained a similar effect, by reflecting fight from 
two metallic mirrors slightly inclined to each other, and whose 
surfaces were nearly in the same plane. The formation of 
these bands depends on the lengths of the paths of two inter- 
fering portions of fight. When the paths are exactly of the 
same length, the two portions of fight will form a brilliant fringe, 
having an intensity greater than that of either portion. If the 
next brilliant fringe corresponds to a difference of paths equal 
to d, then other brilliant fringes will be formed when the dif- 
ferences of the paths are 2d, 3d, 4 d, &c, When the diffe- 
rences of the paths are J d ; d + J d ; 2 d -p J d, or J d, | d, | d, 
the interfering portions neutralize or destroy one another, and 
consequently produce a black fringe. The quantity d has a 
different value for the rays of different colours, and varies as 
the length of the fits in Newton’s theory. M, Fresnel has 
found d to be 
64 
100,000 
of a millimetre for red fight. 
This beautiful theory of Dr Young is embraced by M. Fres- 
nel. Both these philosophers had ascribed the exterior fringes 
^ See Journal of the Royal Institution, vol, ii. p. 207. 
