The evening telegraph. [volume] (Philadelphia [Pa.]) 1864-1918, 1868-08-04

REFRAcTIoN OF LIdHT.-Dr. Gladstone lately
presented at the Royal Institution of London
the results of some new researches relating to
the refractive indices OF different substances,
showing how the index varies with tempera-
ture. By means of an electric lamp he threw
the yellow line in the spectrum of the metal
sodium on screen, and used hollow prism
filled with the oil of nutmeg n the refraoting
medium. This oil was selected because its
temperature is very readily changed, but any
other oil will answer the purpose. The prism
was fixed at-its angle of minimum deviation,
and the position of the yellow band of the
spectrum was marked by a strip of black
paper. On stirring the oil with heated
metallic rods its temperature was increased
and the yellow sodium line was shown to be
less refracted than before. He had tried a
vast number of liquids, and hence concludes
Ithat all liquids have their index of refraction
diminished by heat. He had also turned his
attention to solids, bat on finding that
the refractive index of crown glass was
not changed by heat he discontinued
his investigation; but Fizean of France
had since discovered that in nearly all
cases the refractive index of solids is increased
by heat the two exceptions noted by him
being crown glass, which is not altered, and
fiuor spar, which has its refractive power di-
minished by heat. DR.. Gladstone and the
Rev. Pelham Dale, while experimenting toge-
ther, had discovered one optical property
which was constant under varying tempera-
lures and common to solid, liquid, and gaseous
conditions, namely, the specific refractive
energy; that is to say, the lefractive index,
minus one, divided by the density. The spe-
cifc energy multiplied by the atomic weight,
of a body will give it its refraction equivalent.
Tables had been computed by the lecturer
showing the atomic weights of various elemen-
tary substances and their corresponding re-
fraotive equivalents, which were then exhibited.
Compound bodies generally possess the mean
retracting power of their constituents. By ex-
pertinent it was shown that on mixing spirits
and water the resulting liquid gives little
more refraction than either of these sub
stances before mixture, owing to slight con-
traction in bulk when spirits and water are
mixed, the increased refraction being dne
entirely to increased density. The most gene
raj law arrived at from these researches is that
the refraction equivalent of any substance
(whether mixture or a true chemical COM
pound) is the sum of the refractive equiva-
Tents OF its constituents. The substance of
greatest dispersive power now known is melted
phosphorus. Next to it stands oxychloride OF
vanadium, then follows sulphur. 1t might be
asked why some substances, such as Iceland
spar, are double refractingH DR.. Gladstone
believed such substances have different degrees
OF tension in different directions. The atoms
may be more free to move in one way than in
another, so that in one direction they offel
more resistance to the passage of light. Con
sequently on dissolving such substances the
property of double refraction should disap'
pear. Such on experiment is found to be the
fact, the solutions of such crystals give only
one image, which is about the mean of the tive
seen in the solid. One of the most beautiful
experiments made by DR.. Gladstone was the
formation of ice flowers. slab OF ice GU
parallel to the plane of freezing was placed in
front of the electric light, and greatly magni
fed by means of lens. As the ice began tt
melt the flowers began to appear, growing
like stars on the screen. AS they were en-
larged circle or oval appeared in the middle
of each star. These beautiful and symmetrical
flowers become visible because water refract
light differently from ice. But as water be
tween 32 and 80 degrees Fahrenheit is les
dense, and consequently occupies less spac
than the ice from which it is made, vacuum
is formed in the middle of each star by vrtu
Of the melting ice, which is the cause OF tn
circular or oval appearances.