202 FRESNEL. 
required of experimenters to 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 hardly © 
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 
