246 FRESNEL. 
strike it singly, while on the contrary no such combina- 
tion can take place when these same rays strike it 
together, after having pursued (for this condition is 
necessary) routes differing from one another by quan- 
tities comprised within a certain regular series of num- 
bers ? 
In geometry, in order to demonstrate the inaccuracy ; 
of a proposition, we follow it out to all its consequences 
: 
until there results something which is completely absurd. 
Ought we not to class in this category a chemical action 
which is generated, or which disappears, according to ~ 
the length of route which the reagent has gone through ? 
Natural phenomena ordinarily present themselves un- 
der very complicated forms, and the true merit of the 
experimenter consists in disengaging them from a multi- 
tude of accessory circumstances which hinder us from at 
once seizing their laws. 
If, for example, we had not observed the shadows of 
bodies except in the open air, if we had never illuminated — 
these bodies by light proceeding from extremely small 
luminous points, no one would- have guessed how many 
curious subjects of research are offered by a phenomenon 
so common. But place in the middle of a dark room, 
and in a beam of homogeneous. light, diverging either 
from a minute hole, or from the focus of a glass lens, any 
opaque body whatever, and its shadow will show itself 
marked by a series of contiguous stripes, alternately — 
bright, and completely dark. Substitute white light for 
the homogeneous beam, and similar stripes vividly col- 
oured will appear to occupy the place of the former. 
Grimaldi first perceived these singular affections of 
light, to which he gave the name of diffraction. Newton 
afterwards made them the subject of a special investiga- 
