SlO ANNUAL, REPOET SMITHSONIAN INSTITUTION, 1910. 



radiated from it due to such matter at a high temperature. The nebular 

 hypothesis of Laplace at the end of the last century required, indeed, that 

 matter similar to that of the earth should exist throughout the solar system; 

 but theu this hypothesis itself needed for its full confirmation the independ- 

 ent and direct observation that the solar matter was terrestrial in its nature. 

 This theoretical probability in the case of the sun vanished almost into thin 

 air when the attempt was made to extend it to the stellar hosts ; for it might 

 well be urged that in those immensely distant regions an original difference 

 of the primordial stufC as well as other conditions of condensation were pres- 

 ent, giving rise to groups of substances which have but little analogy with 

 those of our earthly chemistry. * * * 



The darli lines were described first by Wollaston in 1792, who strangely 

 associated them with the boundaries of the spectral colors, and so turned 

 contemporary thought away from the direction in which lay their true sig- 

 nificance. It was left to Fraunhofer in 1815, by whose name the dark lines 

 are still known, not only to map some 600 of them, but also to discover similar 

 lines, but differently arranged, in several stars. Further, he found that a 

 pair of dark lines in the solar spectrum appeared to correspond in their posi- 

 tion in the spectrum, and in their distance from each other, to a pair of 

 bright lines which were nearly always present in terrestrial flames. This 

 last observation contained the key to the interpretation of the dark lines as 

 a code of symbols, but Fraunhofer failed to use it; and the birth of astro- 

 physics was delayed. An observation by Forbes at the eclipse of 1836 led 

 thought away from the suggestive experiments of Fraunhofer; so that in the 

 very year of the Queen's accession the knowledge of the time had to be summed 

 up by Mrs. Somervillb in the negation : " We are still ignorant of the cause 

 of these rayless bands." 



Later on the revelation came more or less fully to many minds. Foucault, 

 Balfour, Stewart, Angstrom prepared the way. Prophetic guesses were made by 

 Stokes and by Lord Kelvin. But it was Kirchhoff who, in 1859, first fully 

 developed the true significance of the dark lines; and by his joint work with 

 Bunseu on the solar spectrum proved beyond all question that the dark lines in 

 the spectrum of the sun are produced by the absorption of the vapors of the 

 same substances, which when suitably heated give out corresponding bright 

 lines; and, further, that many of the solar absorbing vapors are those of sub- 

 stances found upon the earth. The new astronomy was born. 



Soon after the close of 1862, in collaboration with Dr. W. A. Miller, I sent a 

 preliminary note to the Royal Society, " On the lines of some of the fixed stars," 

 in which we gave diagrams of the spectra of Sirius, Betelgeux, and Aldebaran, 

 with the statement that we had observed the spectra of some 40 stars, and also 

 the spectra of the planets Jupiter and Mars. It was a little remarkable that on 

 the same day on which our paper was to be read, but some little time after it 

 had been sent in, news arrived there from America that similar observations on 

 some of the stars had been made by Mr. Rutherford. A very little later similar 

 work on the spectra of the stars was undertaken in Rome by Secchi and in 

 Germany by Vogel. 



In February, 186.3, the strictly astronomical character of the observatory was 

 further encroached upon by the erection, in one corner, of a small photographic 

 tent furnished with baths and other appliances for the wet collodion process. 

 We obtained photographs, indeed, of tlie spectra of Sirius and Capella ; but 

 from want of steadiness and more perfect adjustment of the instruments, the 

 spectra, though defined at the edges, did not show the darli lines as we expected. 

 The dry collodion plates then available were not rapid enough ; and the wet 

 process was so inconvenient for long exposures, from irregular drying, and 



