480 REPORT — 1894. 



character. The sense of the motion is from west to east. The velocity is 

 apparently very variable, and it seems as if we are now approaching an 

 epoch in which the amplitude is considerably diminishing. It is also 

 evident that such a character of movement can very easily produce slow 

 progressive motions, and for this reason the whole phenomenon wants to 

 be watched incessantly and very carefully. 



The astronomers and geodetists who are now associated in the Inter- 

 national Geodetic Union have invited geologists to associate with them 

 in this common research. Such an international organisation will be also 

 useful and almost indispensable for a great part of the work of astronomical 

 observatories. 



It is to be hoped that Great Britain will now participate in this 

 International Union, embracing all other civilised nations. Such organisa- 

 tions, with their clear and reasonably limited aims, involve not only real 

 economies and refinements of mental work, combined with diminutions of 

 material expenses, but it is hoped that they will also have great importance 

 as slowly growing foundations of human and terrestrial solidarity. 



A Lechtre-room Experiinent to illustrate Fres7iel's Diffraction Theori/ 

 and Babinefs Principle. Bij Professor A. CoRNU, F.R.S. 



[Orrlered by the General Committee to be printed in cxtcnso.^ 



The diffraction fringes bordering the shadow of an object illumined by a 

 point of light present us with one of the most striking phenomena in 

 optics. 



Dr. Thomas Young was the first to connect these fringes with the 

 wave theory. According to him they were due to the interference of 

 the direct wave with the wave tangentially reflected at the surface of the 

 body screening the light. 



Fresnel, following the way opened by Young, proved, however, by a 

 very simple experiment, namely, by the identity of the fringes produced 

 by the edge and by the back of a razor, that the reflected light has no 

 appreciable influence on the production of these fringes. He proved that 

 the phenomenon is exclusively due to the mutual interference of all the 

 vibrations proceeding from the whole of the wave not intercepted by 

 the object. 



A mathematical investigation, and even a superficial analysis, shows 

 that the resulting vibratory motion producing the fringes is equivalent to 

 the vibration which would be caused by a permanent wave fixed at the 

 edge of the body screening the light, provided that the acting part of this 

 wave is reduced to a small breadth variable according to the obliquity of 

 the rays. Consequently, everything happens as if the source of light 

 were taken away, and the body were surrounded by a true luminous 

 source forming a sort of border round its apparent contour. This result, 

 proved for a single luminous point, is immediately extended to a circular 

 source of light of any diameter considered as composed of points acting 

 independently. 



This optical equivalence is not only a symbolic or geometrical result, 

 but a real physical fact, and the experiment, which I propose to show, 

 gives the most striking evidence of the reality of this source of light. 



