342 Chronicles of Science. [ April, 
Vis OPRDICS: 
Sinor the beautiful researches of Faraday on gold-leaf, the relation of 
metals to light has scarcely met with the attention which so important 
a subject deserves. M. G. Quincke has recently published* an elabo- 
rate investigation on the optical properties of metals. We have not 
space even for an analysis of this long paper, but we will mention a 
few of the most important results at which he has arrived. Plates of 
gold, silver, and platinum were employed, so thin as to be transparent, 
and these were examined in the same way as other transparent bodies. 
When light falls upon a thick plate of metal it penetrates to a depth 
which is about as great as the length of an undulation, the so-called 
metallic lustre being produced by the conjoint action of the exteriorly 
and interiorly reflected or dispersed light. The velocity of light through 
metals is one of the subjects studied by the author, and he has obtained, 
in the course of this investigation, the remarkable result that light travels 
faster through gold and silver than through a vacuum. But Faraday 
has shown that silver and gold films occur in different modifications, 
and M. Quincke finds that gold and silver metallic plates, through 
which light passes with a greater velocity than through air, may 
become spontaneously altered by simple standing, so as to transmit 
light with less velocity than it is transmitted by air. In the case of 
platinum it was always found that the light passed through with 
less velocity than through air. The ordinary polished silver and gold 
possess the same character as that modification of these metals which 
transmits light with the greater velocity. Their refracting indices are 
therefore less than unity. 
The second part of Kirchhoff’s researches on the solar spectrum 
and the spectra of the chemical elements, translated by H. G. Roscoe, 
F.R.S.,f has just been published. It completes the Professor’s survey 
of the solar spectrum, and contains two plates, one extending from A 
to D, and the other beginning at the point where the second plate in 
the former publication ended, and extending as far as G. The actual ob- 
servations have been taken by M. K. Hofmann, a pupil of Professor 
Kirchhoff’s, his own eyesight having been too much injured by his 
previous observations to allow him to continue the investigation. The 
new metals examined consist of potassium, rubidium, lithium, cerium, 
lanthanum, didymium, platinum, palladium, and an alloy of iridium and 
ruthenium. These additional observations have not yielded any new 
information respecting the constituents of the solar atmosphere ; they 
have, however, confirmed the results of the previous examination. 
Potassium, which was formerly considered to give lines identical with 
some in the solar spectrum, now appears to be absent from that lumi- 
nary; a few coincidences have also been observed in the spectra of 
strontium and cadmium, but their number is too small to warrant the 
conclusion that these metals are present in the sun’s atmosphere. 
* «Pogeendorff's Annalen,’ vol. exix. part 3. 
+ Macmillan and Co, 
