CHAP. V., 5.] 



OPTICS. SIR D. BREWSTER FRAUNHOFER. 



117 



(537.) 

 Singular 

 action of 

 nitrous 

 acid gas 

 on light. 



(538.) 

 Fraunhofer 

 Lines of 

 the spec- 

 trum. 



(539.) 

 Their num- 

 berand im- 

 portance. 



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. 1 When abeam, 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. 



JOSEPH FRAUNHOFER, born in Bavaria, of humble 

 parents, in 1787, raised himself by his unassisted 

 efforts to be the first practical optician of the day. 

 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. 2 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. 



Fraunhofer' s beautiful map of the spectrum, tra- 

 versed by lines of every grade of darkness, and clus- 

 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 



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 to do. 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 dispersiveness in bodies 

 which have been partly already, and will yet much 

 more become, tests of some of the more obscure and 

 diflicultportions 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 a^d'of'the 

 perceived : for, in the first place, it so far accounts earth's at- 

 for the strange phenomenon of the deficient rays mosphere 



of the sun's light, by showing that it may be caused on ^ e 

 , , ,. J . , . J . spectrum. 



by a gas resembling nitrous acid gas in its proper- 



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 

 D. Brewster's, which has received less general assent 



than any other of the numerous and important ones the spec- 

 _ __ _ trum. 



(541.) 

 David 



1 Edinburgh Transactions, vol. xii. (1833). 



8 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. 



