Evening star. [volume] (Washington, D.C.) 1854-1972, 1890-02-15
CITY AND DISTRICT.
WHAT A RAY OF LIGHT TELLS.
How the Sun is Brought Into the Scien.
lists Laboratory and Analyzed.

How the Sun is Brought Into the Scien-
lists Laboratory and Analysed.
TIE sPEcrRoscoPE AND ITS APPLICATION TO SOLAR
| PHYsIcs-PRoF. sKARLEs FRIDAY LECTURE AT
THE CATHOLIC UNIyERsITY-THE MYSTERY OF
| DARK LINES IN THE SOLAR SPECTRUM.
almost every one knows what is meant by
the prismatic colors," said Prof. SearIe in his
lecture ON astronomy at the Catholic university
yesterday afternoon, land those who do not
Know have nevertheless seen them in every
rainbow. II We pass beam OF light through a
prism 1t undergoes remarkable change, both
In color and form. If the beam is circular in
form We will find that after it has been pro-
)ected through the prism upon a screen it ap-
pears as a band eight or ten times as long as it
Is wide. and its color, originally white, now
varies through its whole length by gradual
! shading from a deep red to a dark violet, pass
| ing On its way through the colors orange. yell
| low, green, blue and dark blue or indigo. This
| band Is called the spectrum; il made by sun
night the solar spectrum. In our expert.
| ment no part of the ray Of light
will be in the place where our
beam of light would have struck the
| screen If We had not passed it through the
prism. The rays have all been bent away from
l the euge OF prism; II we mold the PRISM
/ edge down they will be bent upward; If the
i edge is held up they will be bent downward.
| Some Of the rays are bent more than others,
and 1t is this which causes the elongation Of
| the beam and the separation of the colors This
! bending Is called refractiOn ray of light of
/ any color when passing from one medium to
| another, as from air 10 glass or lass TO air, is
| bent or refracted more or less in passing from
one to the other, unless 1t strikes perpendicu-
| larry on the surface separating the two. When
II ~
as from air to glass, it is bent toward the per-
pendicular; when 1t passes from the dense to
| the rare, away.
WAVES AND VIBRATIONS OF LIGHT.
1.-THE objective or physical difference between
two rays of light in different parts of the spec
trum Is precisely the same as that between two
musical notes in different parts of the scale,
/ and consists in a different rate of vibration.
Musical vibrations vary from 600 to 80, 600 in
| second, but the vibrations of light are much
| more numerous, so much more that their num-
Der seems incredible. The vibrations of red
light are at the rate of 453 trillions in a second;
| OF violet at the rate OF 677 trllhons. The aver
age wave length IS about one forty-seven
thousandth of an inch. that of the red greater,
of the violet less, but travels 1856 600 miles
| In second; multiplying the two numbers to-
gether we get the number Of waves of light
which must enter the eye III a second 1t we
make an instantaneous exposure with photo
|
second, III that brief time ray of light 1000
miles long slips into the lens and acts on the
plate with trilions of vibrations. That ought
| 10 be enough to do something; so instantane-
Gus photography Is not so wonderful after all. ~
THE sPECTRosCoPE.
Prof. Searle then proceeded to describe the
mechanism Of the spectroscope and the princi-
piles upon which it is constructed. 1t is, in
short. an instrument composed of prism and
lenses so arranged as to separate beam Of
| light which may be directed through il into its
constituent rays. showing each color separately.
When the slit at the emo OF the spectroscope is
made very narrow, as 1t must be for good OBJ
servations. e great number of dark lines are
seen crossing tho spectrum, all parallel to each
other and each one corresponding to some par
ticuar shade or wave length Of color. These spe-
cial wave lengths of color are 1t were, blotted
out What has blotted them out? That was
Tor long time a mystery. You will not see
| these lines in the spectrum Of the lime light.
| no matter how narrow you make the slit; but
you will see them in the sun and in all objects
illuminated by the sun.
THE MYSTERY OF THE DARK LINES
1t was not till the year 1859 that the answer
was given to this question. Kirchkoff discov-
cred in that year that if the light of burning
sodium We're allowed to fall on the slit instead
OF the light of the sun. bright line was seen
| IN the place Of the dark line ID" in the solar
spectrum; but If the suns light were passed
through this vapor of burning sodium the
dark line looked darker than ever; and, fur
Their. that if the salt of sodium or common salt,
which contains sodium. were burned in front of
the lime light the spectrum Of the lime light,
which of itself has no dark line, will show the
dark line d. Immediately.
The same phenomena were found to occur
with other substances when in the state OF gas
or vapor; and from the researches which have
been made upon them the following laws have
been obtained, which are the foundation of all
spectrum analysis:
l. Incandescent solids and liquids give con-
tlnuous spectra, without lines OF any kind
H. lncandescent gases and vapors give, I'M
der ordinary conditions, spectra consisting Of
isolated bright lines.
III. Gases and vapors through which light is
transmitted absorb from that light lust the
same rays they would themselves emit if lum-
nous.
If an incandescent gas is put in front of
bright solid or liquid substance, SO that We see
hat substance through the gas, what will be
Tue result: The gas is giving. according to the
| secouu u the above laws, a spectrum of bright
lines. But by the third, it is absorbing the
| hight OF the continuous spectrum given by the
| substance behind it precisely in the places of
| those lines One effect. then, might be ex-
| pected to extnguish the other, but ordinarily
| such is not the case. This depends upon the
relative intensity of the two lights. The rule
| Is found to be that the gas does not absorb
precisely the light which it gives out but a
certain and pretty large proportion of the
| light behind it, which is usually much brighter
| than Its own. The result is that ordinarily we
| see dark line IN the spectrum.
GASES AROUND THE SUN.
Now we are in position to understand what
| these dark lines in the solar spectrum are
There must be gas between US and the bril-
| liant surface Of the sun which is absorbing the
| llSnt which that surface gives. That surface
| may be itself gaseous in one sense: that il not
\ entirely reduced to the liquid or solid state. in
| either of which il would remain of itself in its
| condensed condition without external force
| whereas If a gas it would expand were the
| pressure removed; but whether it be 40 or not
| is immaterial, for gas reduced by great press
| ure to the appearance of liquid or solid gives
a continuous spectrum like real liquid or solid
| This surface then gives a continuous spectrum
| special lines of which are absorbed by the gas
| more truly such, above il, which is at a lower
| temperature and pressure, and thus we have
the dark lines.
In order to find out what these dark lines
| represent all we have to do is to let the light
OF the sun cover half Of the slit in the spectre
scope and into the other half send the light of
| some lncandescent vapor. which NAV be OBJ
tanned by volatilizing the metal the spectrum
| Of which is desired in the great heat of the
electric arc. BY this means we obtain two
| spectra side by side.
We now see that certain dark lines in the
solar spectrum correspond exactly in position
| with the bright lines which this vapor gives:
| thus we have discovered one of the elements of
which this gas about the suns surface is com