92 
POPULAR SCIENCE REVIEW. 
of a spectroscope the image of Babinet’s compensator, arranged to give 
horizontal lines, and causing it to be traversed by two luminous pencils, rec- 
tangularly polarized, producing a spectrum marked with horizontal lines. 
Each pencil gives a separate system of lines, and the dark portions of one 
being superposed on the bright parts of the other, it is always possible, by 
varying the intensity of one light by a known quantity to extinguish the 
lines in one portion of the spectrum. The apparatus is arranged as follows : 
on the axis of the collimator are placed the following pieces, — An achromatic 
lens ; a Nicol’s prism, with its principal axis horizontal ; a Babinet’s com- 
pensator, of peculiar form ; an achromatized prism of Iceland spar ; a Nicol 
movable in graduated circle, and a lens. Between the last Nicol and the 
spar prism, the tube supports a lateral branch at a right angle, containing a 
total reflection prism and a lens. A flame is placed in front of this side branch, 
its rays pass through the prism as extraordinary, whereas the opposite occurs 
with flames placed in front of the movable Nicol. They thus give comple- 
mentary lines, either series of which can be varied in intensity at will. This 
instrument had the defect of requiring great intensity of light : and it has 
been materially improved. 
The more recent form is that of a two-prism spectroscope, the second 
prism of which is movable round the centre of the table, and is attached to 
the arm bearing the telescope. Before the object-glass of the collimator is 
a flat mirror covering its upper half. On this are reflected the rays coming 
from a second collimator, so that if before the two collimator slits are placed 
two identical sources of homogeneous light, there will be seen the two images 
coincident in the focus of the observing telescope. There will, therefore, be 
two semicircles of unequal brilliancy. To equalize them the second collimator 
has two Nicol’s prisms in its axis, one fixed, the other movable : a slit is 
substituted for the eye-piece of the observing telescope. The coloured flame 
to be studied is placed before the first-named collimator, and the standard 
flame before the second. 
A method of producing constant coloured flames by means of pulverized 
saline solutions, forming part of the necessary apparatus, is described at 
length. Observations follow on sodium flames ; on the transparency of 
coloured flames, for their proper and for heterogeneous radiations ; on the 
density of metallic vapours, and on the reducing or oxydizing portions of 
solid conical masses of heated gas. 
Optical power of Spectroscopes . — Lord Rayleigh notes in the Philosophical 
Magazine that as the power of a telescope is measured by the closeness of 
the double stars which it can resolve, so the power of a spectroscope ought 
to be measured by the closeness of the closest double lines in the spectrum 
which it is competent to resolve. In this sense it is possible for one instru- 
ment to be more powerful than a second in one part of the spectrum, while 
in another part the second instrument is more powerful than the first. The 
most striking cases of this inversion occur when one instrument is a 
diffraction- and the other a dispersion-spectroscope. If the instruments 
are of equal power in the yellow region, the former will be more powerful 
in the red and the latter in the green. That the resolving power of a 
prismatic spectroscope of given dispersive material is proportional to the 
total thickness used, without regard to the number, angles, or setting of the 
