On some Nexo Lines in the Solar Spectrnm. 499 



Hence 



dt- pdz pdz ^ ^' pdz dz 



The usual theoiy explains how the third term of the right- 

 hand side of this equation may be in a given ratio to the first; 

 but my difficulty is to conceive how the same can be the case 

 also with the second term, since it changes sign with the change 

 of sign of 9 — flj. 



Cambridge Observatory, 

 May 19, 1848. 



LXVI. On some Ne'w Lijies in the Solar Spectrum. 

 By Professor Elie Wartmann*. 



TT^HE study of the spectrum has long occupied the attention 

 J- of philosophers. As early as 1802 Wollaston made 

 known the existence of some obscure rays perpendicular to its 

 length. Twelve years later Fraunhofer again discovered them, 

 and gave an analysis of them, which is a masterpiece of patience 

 and skill. We know that he used these lines as extremely 

 valuable marks for the construction of achromatic refractive 

 systems. Having been sought for in spectra produced with 

 different sources of illumination, and with prisms and absorb- 

 ing media of various kind, they have likewise become of con- 

 siderable interest, as affording geometricians an opportunity 

 of discussing the comparative merits of the theories of light. 



In 1840 I made a series of experiments on the spectrum 

 with an excellent prism of flint-glass, the last cut by the illus- 

 trious Bavarian optician. These experiments led me to several 

 results, which I conmiunicated to my pupils and showed to 

 some amateurs ; but I desired to increase their number 

 before publishing them in a connected form. At present, not 

 having the apparatus which I employed any longer at niy 

 disposal, it is useless to make further delay. Notwithstanding 

 their imperfection, I may be allowed to make known some 

 details that I believe to be new. I extract them briefly from 

 the note-book of my laboratory, dated April and May 1841. 



" The lecture-room of the Academy of Lausanne having 

 been darkened, a beam of solar light, thrown by a tinned or 

 blackened mirror, was made to pass through a vertical slit 

 of Omm.3 i„ width. This beam is refracted in Fraunhofer's 

 prism fixed in a vertical position at the distance of eight me- 

 tres. Directly behind this instrument is placed an aclnomatic 

 theodolite of 0'"*04 aperture, made at Aarau, or a comet seeker 

 • Communicated by the Author. 

 2 K2 



