Carvallo has noted in some extremely careful
experiments, the dispersion of the infra-red spectrum, and not at all
the peculiarities presented by absorbent substances.
M. Boussinesq arrives at almost similar results, by attributing
dispersion, on the other hand, to the partial dragging along of
ponderable matter and to its action on the ether. By combining, in a
measure, as was subsequently done by M. Boussinesq, the two
hypotheses, formulas can be established far better in accord with all
the known facts.
These facts are somewhat complex. It was at first thought that the
index always varied in inverse ratio to the wave-length, but numerous
substances have been discovered which present the phenomenon of
abnormal dispersion--that is to say, substances in which certain
radiations are propagated, on the contrary, the more quickly the
shorter their period. This is the case with gases themselves, as
demonstrated, for example, by a very elegant experiment of M.
Becquerel on the dispersion of the vapour of sodium. Moreover, it may
happen that yet more complications may be met with, as no substance is
transparent for the whole extent of the spectrum.
Pages:
364
365
366
367
368
369
370
371
372
373
374
375
376
377
378
379
380
381
382
383
384
385
386
387
388