The times. [volume] (Washington [D.C.]) 1897-1901, 1901-02-17

| physical science.
The work of Thomas Young was done ii
the very first years OF the nineteenth cen
fury. lie was for two years professor o
natural philosophy in the Royal Institu
ton just founded 10y Count Rumford, an,
ne was the first TO fill that chair. in 1901
In paper presented to the Royal Society
ne argued In favor OF the undulatory the
DRY, showing how the interference O
waves would explain the color of thir
plates His papers were not, for severa
years. received favorably. and they wer,
severely criticised by Lord Brougham
Augustus Fresnel followed Young, but
quite independently, about ten years later
ana by him the undulatory theory received
elaborate experimental and mathematics
treatment.
In the mean time another Frenchmar
had made capital discovery In optics
which seemed al first TO be quite incom-
patible with the wave theory. This was
the discovery OF what is known as polari-
cation of light by Maus. n French en-
gineer, who hit upon 1t while investlgat-
ing double refraction of crystals for a
study of which the French Institute had
offered prize in 1905. Malus found that
when light fell upon a surface of glass
at a certain angle n portion of the re-
fiected light appeared TO have acquired
| entirely new properties In regard to fur
| Their reflection, and the same was true
| of that part of the beam which was trans
| mitted through the glass. The light thus
| affected was incapable of further refiec-
| ton under certain conditions and as the
beam seemed to behave differently ac-
| cordipg TO how II was presented to the
| rejecting surface, the term polarization
was applied to the phenomenon. 1t was
| found that the two rays into which a
single beam of light was split by a doubly
refracting crystal Ia phenomenon which
| had long been known were affected in
| this way. and that light was polarized by
| refraction as well as by reflection. Malus
| was a believer in the corpuscular theory
| of light, but II was shortly proved, first
| by Thomas Young, that the phenomenon
| OF polarization was not only not opposed
| to the wave theory but that that theory
| furnished a rational explanation Of it.
This explanation, In brief, assumes that
ordinary light is wave produced by
vibratory motion confined to no particu
lar plane the direction of vibration being
at right angles to the direction of the
wave and in any or, in rapid succession,
in all azimuths. When light is polarized
the Migratory motion In the ether Is
stricted TO one particular form, line II
plane polarized, circle or an ellipse, IL
circularly elliptically polarize-:. This
simple hypothesis has been found quite
adequate, and through its application to
the various phenomena Of polarization. to-
gether with the application of Youngs
theory Of the Interference of waves TO the
production OF color the undulatory theory
OF light was firmly established before the
middle of the century, There were many
noted philosophers. however, who stood
out long against It, notably Brewster, the
most famous English student OF optics OF
the early part OF the century, who de-
clared that his chief objection to the
undulatory theory, was that he could not
think the Creator guilty of so clumsy a
Continuance as the filling of space with
ether In order to produce light In study
ing the nature of light II became very i'm
portant to know how fast a light wave
travelled. A tolerably good measure of
the velocity OF light had been made long
before by means Of the eclipses of Jupi-
ter's moons and by observations upon the
positions OF the stars as influenced by the
motion OF the earth IN its orbit. II was
found to be approximately 10000 miles
per second, speed so great that It
seemed impossible that 1t should ever be
measured by using only terrestrial dis
tances.
This extremely difficult problem has
heap in
tory manner by nineteenth century phys |
icists. Everybody knows that in a uni |
form nation velocity Is equal t. space or |
distance divided by time. II, then, the |
time occupied in passing through a given |
distance can be measured the velocity is |
at once known. AS the velocity of light |
is very large unless the distance is enor- |
mously great, the time will be extremely |
small and IL moderate distances are to be
used the problem Is to measure very
small intervals OF time very accurately
Light will travel one mile in about the
one hundred and eighty-six thousandth |
part of second, and II by using a mile
as the distance the velocity OF light Is to
be determined within 1 per cent, 1t Is
to be able to detect differences
OF time as small as about one twe.Ity-mil-
lionth Of second. This has been made
possible by the use of two distinct meth
DDS. Foucault. on the suggestion of Arago, |
used rapidly revolving mirror, a method
introduced by Wheatstone, the English
who used # in finding the du-
ration Of an electric spark. The essential
principle is that mirror may be made to
revolve so rapidly that 1t will change Its
eli nv mag 10 ,.,lI 1
which has been reflected from 1t passes to
a somewhat distant fixed mirror and re-
turns to the moving reflector. In the other
method a toothed wheel is revolyed SO
rapidly that beam of light passing be-
tween two consecutive teeth to distant
fixed mirror is cut OFF on its return to |
he wheel by the tooth. which has moved
forward while the light has made its Jour
ney. This method was first used by
Fizeal. In either method, il the speed of
rotation Is known. the time is readily |
'ound. In point of time Fizeau was the
irst to attack the problem which he did |
ibout 1Si9. Foucault was perhaps a year
ater in getting results, but his method is |
considered the best. Both meth
IDS have been used by other experiment.
RS, and very important improvements In |
Toucaults method were made in the |
United States by Nicholson about 1878.
method. increased enormously
he precision of the measurements, and IT /
las been applied by him and by Newcomb
lot only for the better determination of !
he velocity of light In air, but for the
olution of many other related problems
# first Importance. Mchelson's final de- |