TRANSACTIONS OF THE SECTIONS. 15 
On a New Micrometer. By M. H. Soret; exhibited and explained 
by the Abbé Moteno. 
This consisted of two Ramsden’s eyepieces, one fixed near the object or image 
to be measured, the other moveable to suit the vision of the observer, with a ruled 
glass micrometer-plate placed between them. The magnifying ated of this eye- 
piece being ascertained by a comparison of the object as seen directly, with the 
same object as seen through the micrometer, it then became applicable to the 
telescope, the microscope, and even to goniometry by a certain adjustment, and 
haying the plate to which the objective eyepiece was attached graduated on its 
circumference, 
On Spectral Analysis. By Prof. Pritcxer. 
It is generally admitted now, that every gaseous body rendered luminous by heat 
or electricity sends out a peculiar light, which, if examined by the prism, gives a 
well-defined and characteristic spectrum. By such a spectrum, by any one of its 
brilliant lines whose position has been measured, you may recognize the examined 
gas. This way of proceeding constitutes what is called spectral analysis, to which we 
owe, already, the discovery of three new elementary bodies. In order to give to 
spectral analysis a true and certain basis, you want the spectrum of each elementary 
substance. Very recently, some eminent philosophers, in examining such spectra, 
met with unexpected difficulties, and doubts arose in their minds against the new 
doctrine. Those doubts are unfounded. The factis, that the molecular constitution 
of gases is much more complicated than it has been generally admitted to be till now. 
The spectra therefore, always indicating the molecular constitution of gases, must 
also be more complicated than they were at first thought to be. By these consider- 
ations, a new importance, of a physical nature, is given to spectral analysis. You 
may recognize by the spectrum of a gas, not only the chemical nature of the gas, but 
you may also obtain indications of its more intimate molecular structure—quite a 
new branch of science. Allow me now to select, out of the results already obtained, 
two instances only. Let me try to give what I may call the history of the spectra 
of two elementary bodies—of sulphur and nitrogen. In order to analyze by the 
prism the beautiful light produced by the electric current if it pass through a 
rarefied gas, I gave to the tube in which the gas is included such a form that its 
middle part was capillary. Thus I got within this part of the tube a brilliant film 
of light, extremely fitted to be examined by the prism. The date of my first paper 
on this subject is the 12th of March, 1858. About a year ago, after haying pro- 
vided myself with apparatus more suited to my purpose, I asked my friend Professor 
Hittorf, of Miinster, to join me in taking up my former researches. The very first 
results we obtained in operating on gases of a greater density opened to us an im- 
mense field of new investigation. We found that the very same elementary sub- 
stance may have two, even three, absolutely different spectra, which only depend 
on temperature. In our experiments we made use of Ruhmkorff’s induction-coil, 
whose discharge was sent through our spectral tubes. In order to increase at 
other times the heating-power of the discharge, we made use of a Leyden jar. 
Now, let us suppose a spectral tube, most highly exhausted by Geissler’s mercury- 
pump, contains a very small quantity of sulphur. The discharge of the coil will 
not pass through the tube if it do not meet with ponderable matter, either taken 
from the surface of the glass or, if the discharge be very strong, by the chemical 
decomposition of the glass. In heating the tube slowly by means of a lamp, in 
order to transform a part of the sulphur into vapour, all accidental spectrum, if 
there be one, will disappear, and you will get a pure and beautiful spectrum of 
sulphur. I suppose the Leyden jar not to have been interposed. you now 
interpose it, the spectrum just spoken of will suddenly be replaced by quite a dif- 
ferent one. We were, generally, led to distinguish two quite different classes of 
spectra. Spectra of the first class consist in a certain number of bands, variously 
shadowed by dark transversal lines. Spectra of the second class consist in a great num- 
ber of most brilliant lines on a dark ground. Accordingly, sulphur has one spectrum 
of the first class and another one of the second class. You may, as often as you 
like, obtain each of these two spectra. In operating on a spectral tube containing 
