ESSAY ON THE VELOCITY OF LIGHT. 
BY M. DELAUNAY, OF THE INSTITUTE OF FRANCE. 
TRANSLATED FOR THE SMITHSONIAN INSTITUTION BY ALFRED M. MAYER, PROFESSOR OF PHYSICS, PENN- 
SYLVANIA COLLEGE, GETTYSBURG. 
Tue precise determination of the velocity with which light traverses space, 
first accomplished by the aid of astronomical observations, and then reduced 
to the proportions of a simple experiment of physics, made in a laboratory of 
small dimensions, constitutes certainly one of the wonders of modern science. 
We propose to exhibit in this essay the different phases through which the 
problem has passed, and the various methods which have been successively 
employed to arrive at its solution. 
We will first endeavor to convey a clear idea of what is understood by the 
velocity of light. 
In order to account for the various phenomena of optics, physicists have 
imagined two systems* in reference to the ultimate nature of light. According 
to one, every luminous body throws out contjnually and in all directions into 
space corpuscles of an extreme tenuity, which in penetrating our eye there 
produce the sensation called light; this is the system of emission. According 
to the other, light is the vibratory motion of an excessively rare fluid existing 
in all space, and known under the name of ether ; a luminous body only pro- 
duces and keeps up around itself this vibratory movement which is progres- 
sively propagated to an indefinite distance ; this constitutes the system of undu- 
lation. Can we certainly say that either system is a true expression of what 
exists? We cannot. Fora long time one system as well as the other ac- 
counted for all known optical phenomena; now we know that certain phe-_ 
nomena that we have succeeded in producing cannot be explained by the sys- 
tem of emission, whilst the system of undulation logically embraces them; 
the latter, therefore, of the two can alone be the true expression of the facts. 
But it may happen that at some future day we will discover new phenomena 
which will be no more explicable by the system of undulation than those men- 
tioned can be by the system of emission. But however that may be, it is 
necessary, in order to give clearness and precision to our reasoning, to represent 
the phenomena as taking place according to one or the other system, provided 
we do not absolutely give our mental consent to such a system as being a true 
expression of existing facts. 
In the system of emission the luminous corpuscles thrown out in all diree- 
tions are in reality projectiles which move uniformly and in straight lines, 
provided they are not submitted to causes of change of motion, such as those 
which result from the meeting of ponderable or gross matter. ‘The velocity 
with which these projectiles move in space is what we call the velocity of light. 
* The word system is used by the author in this essay to designate a body of doctrine, and, 
therefore, may mean either theory or hypothesis. See Prof. J. Henry’s Syllabus of Lectures 
on Physics for the true meaning of these words, which, unfortunately, are so generally mis- 
understood and thoughtlessly used. An hypothesis may be defined as an assumed law, 
whereas a theory is ‘‘ the exact expression of the law of a class of facts,” and may be termed 
a vindicated hypothesis.— Translator. 
