PKESIDEXTIAL ADDRESS SECTION A. 39 



views and so high was his prestige among physicists, that only a 

 very few agreed with Huygens. The tenacity and prestige of 

 Newton weighed heavily upon physical optics, during nearly two 

 centuries, retarding the development of this branch of Science. 



It was only in 1819 that a French engineer, Fresnel, depart- 

 ing from routine', established his wave theory. But Fresnel 

 carried his audacity farther. Instead of, like Huygens, assuming 

 longitudinal displacements, he did not hesitate to assume trans- 

 versal displacements of a medium (the ether) which he first 

 defined as an imponderable and perfect fluid. 



In solid bodies the cohesion of molecules allows us to explain 

 how the movement of a particle in a given direction can draw the 

 particles in a perpendicular direction, giving rise to transversal 

 propagation; in a perfect fluid, such cohesion does not exist at all, 

 nor does the transversal propagation exist. Furthermore, in 

 solid bodies the displacements propagate in all directions, giving 

 rise to both longitudinal and transversal propagations ; in perfect 

 fluids there is only the longitudinal one. In both cases, the 

 longitudinal displacements always exist. 



Fresnel, however, did not assume for the ether tlie existence 

 of such displacements. The great mathematician, Monge, did 

 not hesitate to say that Fresnel's theory was an " absurdity of 

 mechanics." But, if this theory was not in agreement with 

 common sense and the results of experiments made in vibrating 

 solids and fluids, it explained very fairly all known optical 

 phenomena. So physicists were forced to agree with Fresnel's 

 views and were content with assuming for the ether those 

 properties which could keep common sense untouched. 



In order to explain the existence of transversal displace- 

 ments, ether was regarded as an elastic solid. In order to explain 

 the non-existence of longitudinal ones, two ways could be 

 followed : to assume an instantaneous propagation or an infinitely 

 slow one. In the former way, ether was to be regarded as 

 incompressible; in the second one, as having a negative coefficient 

 of elasticity, or, say, a tendency to contract. 



Evidently, the theoiy would not resist so many phantasies, 

 had it not the strong support of numerous and varied experiments 

 of which it gives a sound explanation. 



Later on, Maxwell, comparing data measured in electrostatic 

 and electromagnetic units, arrived at the conclusion that electro- 

 magnetic disturbances propagate with a velocity equal to the 

 ratio of those units, which was found to be equal to the light 

 velocity in vacuo. This was the basis of his theory — the electro- 

 magnetic theory of light — in the development of which he pre- 

 dicted the existence of the waves which have taken the name 

 of the man who, for the first time, produced them in the 

 laboratory— Hertz. It was, therefore, the knowledge of the 

 velocity of light, discovered by the astronomical observations of 

 Roemer, which allowed this remarkable advance of science and 

 civilisation. 



