1870.] 



President's Address. 



121 



"His third memoir (1863) contains a determination of the wave-lengths 

 " of Fraunhofer's principal lines, and others, 70 in all. As the same matter is 

 " treated far more completely in a subsequent memoir, it is only necessary to 

 "notice that he corrects his measures for the pressure and temperature of 

 " the air, for the temperature of the grating employed, and for the aberration 

 " caused by a motion of the instrument in a direction at right angles to the 

 " incident light. This last will cause a difference between the deviations on 

 " each side of zero. In the high latitude of Upsala, and in an unfavourable 

 " summer, the observations which he made to test the reality of this correc- 

 " tion were neither sufficiently good nor numerous to satisfy him ; yet the 

 " difference which he found agreed very closely with what was computed. 



" A memoir (1865) gives the places and map of the solar lines beyond G, 

 " where Kirchhoff's map terminates. His spectroscope-arrangements are ori- 

 ginal. He uses two 'astronomic telescopes' (probably of 3|-inch aper- 

 " ture) as collimator and observing-telescope, and a single bisulphide-of- 

 " carbon prism of 60°. The large aperture of the telescopes gives illumina- 

 " tion enough to permit the use of a high magnifying-power ; and this compen- 

 sates for a less dispersion*; so that the apparatus showed all Kirchhoff's 

 " lines with great distinctness. For extreme violet rays an image of the sun 

 "was formed on the slit by an object-glass like that of the telescope. The 

 " metallic spectra were obtained, not by an induction-machine, but by the 

 " voltaic discharge of a battery of 50 Bunsens between massive electrodes of 

 " each metal. It may be that in this way a higher temperature is attained 

 " than by the induction-spark (especially if Wilde's instrument were substi- 

 " tuted for the battery) ; for he found that more lines, and of higher inten- 

 " sity, were thus brought out than even by the large Ruhmkorff. 



" He thus found in the solar spectrum 390 iron-lines more than were pre- 

 viously known. He also found lines of manganese, and ascertained the 

 " identity of the line h with the fourth hydrogen-line which he had pre- 

 viously discovered. 



"In 1866, he found that the lines A a B, and some others, though 

 " telluric, are not due to aqueous vapour ; for they are of undiminished 

 " strength at 1 6° below zero of Fahrenheit, while the other telluric lines were 

 "scarcely visible. 



" In 1869 his researches on the solar spectrum contained determinations 

 " of wave-lengths. These were made by means of four gratings by Nobert, 

 " celebrated for his microscopic test-lines. These determinations appear 

 " to be exceedingly accurate. The intervals of the gratings were from 

 ""22IU mc h to T&Tft* an( l were determined with extreme care. 



"They were compared by a microscope of 200 power, carried by a 

 " dividing-engine carefully verified with a metre which had been compared 

 " with the standard platinum one at the Conservatoire des Arts et Metiers. 



" The corrections already mentioned for pressure, temperature, and aber- 



* \Terz found that, with a telescope of 4 inches aperture, and flint prism of the same 

 " dimensions, 60°, the line between the two of D was visible." 



