SCIENTIFIC SUMMARY. 
93 
prisms, is a most important — perhaps the most important — proposition in 
connexion with this subject. Hitherto, in descriptions of spectroscopes, far 
too much stress has been laid upon the amount of dispersion produced 
by the prisms. But this element by itself tells nothing as to the power of an 
instrument. It is well known that by a sufficiently near approach to a 
grazing emergence the dispersion of a prism of given thickness may be 
increased without limit, but there is no corresponding gain in resolving- 
power. So far as resolving-power is concerned, it is a matter of indifference 
whether dispersion be effected by the prisms or by the telescope. Two things 
only are necessary : first, to use a sufficient thickness ; secondly, to narrow 
the beam until it can be received by the pupil of the eye — or rather (since 
with full aperture the eye is not a perfect instrument), until its width is not 
more than one third or one fourth of the diameter of the pupil. 
Dispersion of Dark Heat-rays, and Measurement of their Wave-lengths, 
was, according to M. Mouton, first attempted by Herschel, in 1800; it 
has since been the subject of many researches, which may be divided into two 
groups. The former, excluding all hypotheses as to the nature of the radia- 
tion, endeavours to determine either the position of the heat-maximum, and 
its dependence on the material and thickness of the prism, or the laws of 
absorption and transmission proper to the different parts of the spectrum. 
The latter are so much more important as to need a chronological sketch. 
In 1818 Berard showed that solar heat is polarized by reflection, and under- 
goes double refraction. In 1834 Forbes discovered the action of tourmaline 
and of bundles of mica ; Melloni and Biot, the rotation by quartz of the plane 
of polarization ; Wartmann, that by magnetic force. In 1847, Fizeau and 
Foucault produced interference-phenomena by diffraction; and then added 
the coincidence in the light spectrum, and the continuation in the ultra-red 
of the bright and dark bands, obtained either by a crystal plate or by a 
perpendicular lamina of quartz between two polarizers. The same year 
there appeared the researches of Knoblauch, Provostaye, and Desains, who 
established — (1.) That the two heat-pencils emerging from Iceland Spar are 
completely polarized : one in the principal axis, the other at right angles to 
it. (2.) That polarized heat follows the laws of light. (3.) That the 
variations of intensity, after reflection from glass at different incidences, agree 
with Fresnel’s formula. (4.) That there is perfect resemblance between the 
phenomena of polarized light and heat when reflected from polished metals. 
They also measured accurately the rotation of the plane of heat-polarization 
produced by a magnet. In 1850 they studied the polarizing action of glass 
bundles, and determined the rotatoiy power of turpentine and sugar solutions 
for heat. M. Desains, by means of bundles of rock-salt plates, was able 
to polarize in definite quantity the rays of heat emanating from a source at 
about 300° Cent., which fail to pass through glass, finding them to be con- 
formable to Frosnel’s laws for glass and light. M. Mouton continues what 
he calls the 1 graduation of the heat-spectroscope,’ using a modification of 
M. Fizeau’s method : a plate of quartz, cut parallel to the axis, being placed 
between two parallel Nicol’s prisms. The source of heat was a Bourbouze 
lamp, with a hood of platinum, heated to a white heat by means of gas and 
compressed air. This was placed in a neighbouring room, communicating 
with the other apparatus by means of a lens forming an inverted image at 
