Cuar. V.,§ 5.] 
the transmitted light is made up of different portions 
of the spectrum curiously blended, whilst rays 
intermediate in the order of refrangibility are wholly 
stifled. Many crystals have the curious property of 
dichroism, that is, of transmitting light of different 
colours in different directions. All these facts have 
been very carefully studied by Sir David Brewster. 
But the most remarkable phenomenon to be noticed 
under this head is the wonderful action of nitrous acid 
gas upon light.!' When a beam, either of sunlight, or 
the light of a lamp, is passed through a bottle contain- 
ing a small quantity of fuming nitrous acid, the light 
emerges of a tawny orange colour, which may be 
deepened indefinitely by heating the acid. If this light 
be then analyzed by a common prism, a wonderful 
spectacle is seen. The spectrum appears traversed 
by countless bands or dark spaces, whilst the blue 
and violet colours are nearly absorbed. The effect 
of the gas, then, is this,—to stifle or absorb countless 
minute portions of light seemingly selected at ran- 
dom from every part of every colour in the whole 
spectrum. Some of these deficient rays are broad 
and palpable, but most of them are so fine as to be 
visible only with the telescope. To understand the 
full import of this discovery, it is necessary to 
describe first the lines of Fraunhofer. 
(588.) JosepH Fraunuorer, born in Bavaria, of humble 
Fraunhofer parents, in 1787, raised himself by his unassisted 
Sane, efforts to be the first practical optician of the day. 
trum. He had also the merit of devoting his leisure and 
the fine apparatus at his command to the observa- 
tion and discovery of many optical phenomena, par- 
ticularly those diffractive colours produced by fine 
gratings, which are known under the name of Fraun- 
hofer’s spectra.? His principal discovery, however, 
was (in 1814) that of countless deficient or dark 
lines in the solar spectrum, resembling those which, 
as we have mentioned, were afterwards observed by 
Sir D. Brewster, to be produced in any kind of 
light by the action of nitrous gas. The deficient rays 
of solar light had, indeed, been observed still earlier 
(in 1802) by Dr Wollaston, but he counted only a 
very few of the more conspicuous ones ; he described 
them merely incidentally, and (unusually with him) 
seems not to have perceived the great value of the dis- 
covery both in a theoretical and practical point of view. 
(539.) Fraunhofer’s beautiful map of the spectrum, tra- 
prea versed by lines of every grade of darkness, and clus- 
portance, tered with every conceivable variety of distribution, 
was published in the Munich Transactions. He 
counted 590 lines, but Sir D. Brewster states that he 
has carried the number to 2000. Like the stars, they 
are probably countless. These lines characterize solar 
light. The light of the fixed stars and that of the 
(537.) 
Singular 
action of 
nitrous 
acid gas 
on light. 
OPTICS.—SIR D. BREWSTER—FRAUNHOFER, 
915 
electric spark have their peculiar deficiencies diffe- 
rent from those of our sun, These were disco- 
vered by Fraunhofer, as well as their occurrence in 
certain coloured flames. The order and number 
of the lines is, in each case, independent of the 
kind of prism used; but the angular distribution 
of the deficient rays varies with the material. 
Thus an oil of cassia prism expands most in pro- 
portion the less refrangible end of the spectrum ; 
while water and sulphuric acid act with dispropor- 
tionate dispersive energy on blue and violet light. 
This property of substances had been already studied 
by Sir D. Brewster with his usual diligence; but the 
importance of Fraunhofer’s discovery was this, that 
the lines (the larger of which he distinguished by 
letters of the alphabet) furnish landmarks which de- 
fine special rays of light invariably recognisable under 
all circumstances, which the vague description of 
their tints is quite incompetent todo. This enabled, 
on the one hand, the practical optician to discover 
the kinds of glass most fit for achromatic combina- 
tion; and, on the other, it afforded precise numerical 
measures of the quality of dispersivencss in bodies 
which have been partly already, and will yet much 
more become, tests of some of the more obscure and 
difficult portions of the theory of light, —those, namely, 
which are connected with dispersion and absorption. 
Fraunhofer was, after Dollond, the most eminent and 
scientific manufacturer of achromatic telescopes, of 
which he vastly increased the aperture, He died at 
Munich in 1826. 
Returning to Sir David Brewster’s discovery of the (540,) 
artificial production of analogous lines or deficient Action of 
rays in light from any source, its importance is easily 7° o 
perceived :—for, in the first place, it so far accounts carth’s at- 
for the strange phenomenon of the deficient rays mosphere 
of the sun’s light, by showing that it may be caused - 
by 2 bling nitrous acid gas in its proper- ?°°""™ 
y a gas resembling g prope 
ties existing in the solar atmosphere; and, farther, 
if so astonishing a result of absorption is ever to be 
explained by theory, the first step is to be able to 
produce the phenomenon at pleasure, and to examine 
the qualities of the bodies producing it. The phe- 
nomena of coloured flames which possess standard 
deficient rays, present perhaps a closer analogy to 
the sidereal spectra. Sir David Brewster has far- 
ther found that the absorptive action of the earth’s 
atmosphere (detected by the varying character of the 
spectrum for different angular altitudes of the sun) in- 
creases the number and also the breadth of these lines. 
Intimately connected with, and nearly of the same _ ) 
date as these experiments, was an observation of Sir Had Mite 
D. Brewster’s, which has received less general assent analysis of 
than any other of the numerous and important ones the spec- 
tram, 
(541.) 
1 Rdinburgh Transactions, vol. xii. (1833). 
® The peculiarity of these spectra is this, that they consist of pure colours, whilst almost all interference-colours are, like those 
of Newton’s rings, mixed and impure. One result is very remarkable. Fraunhofer obtained his spectra of such brilliancy as to 
be able to measure the position of the dark lines (an evidence of their exceeding purity), thus obtaining a standard spectrum in 
which the material of the prism has no influence whatever in varying the ratio of the dispersion of the various colours. 
