202 FRESNEL. 



required of experimenters lo be able to saw with a file, 

 and to file with a saw. 



Want of time will not permit me to refer here to 

 other various labours of our colleague equally relative to 

 the refraction of light, and of which I do not exaggerate 

 the importance in saying that they would alone suffice 

 to establish a reputation equal to that of many physi- 

 cists of the first eminence. I hasten to pass on to an 

 optical theory not less interesting, and altogether of 

 modern date ; which is designated by the name of the 

 theory of "Interferences." It will furnish me with new 

 occasions to render apparent the astonishing perspicuity 

 of Fresnel's mind, and the inexhaustible resources of his 

 inventive genius. 



INTERFERENCES. 



The very name of " interference " has as yet hai'dly 

 emerged beyond the precincts of scientific societies, and 

 yet I know not whether any branch of human knowl- 

 edge presents phenomena more varied, more curious, 

 more strange. Let us endeavour to disengage the 

 capital fact which pervades this whole theory of the 

 technical language in which it is commonly enveloped, 

 and we may hope it will before long be admitted that it 

 deserves in a high degree to attract public attention. 



I will suppose that a ray of the sun's light falls 

 directly on any screen, as for instance on a sheet of fine 

 white paper. The part of the paper on which the ray 

 falls will of course be brightly illuminated ; but it might 

 seem incredible if we assert that it depends on the ex- 

 perimenter to render this spot perfectly dark without 

 stopping the ray or touching the paper. 



What then is the magical process which allows us to 



