435 



Artjcle XI. 



On the Spectra of Fraunhofer formed by Gratings, and on the 

 Analysis of their Light. By O. F. Mossotti, Professor of 

 Mathematics in Pisa. 



[From a separate Memoir, entitled Sidle Propriefd degli Speltri di Fraunhofer 

 formati dai Reticoli ed Analisi delta Luce die somminiUrano, Pisa, 1845.] 



JL HIS memoir consists of two parts. The first, which may be 

 regarded as the introduction, contains the notice of the mathe- 

 matical analysis of the solar spectrum, as read in the Physico- 

 mathematical Section of the fifth assembly of Italian natural 

 philosophers held at Lucca. The second part developes the 

 calculus instituted in continuation for the purpose of more accu- 

 rately deducing from Fraunhofer's experiments those results 

 which were merely announced at the commencement of the in- 

 vestigation. 



Part I. — Introduction. 



1. Those philosophers who have examined the solar spectrum 

 with the view of ascertaining the extent of the colours it con- 

 tains, the intensity of the hght at various parts, and the length 

 of the corresponding fits or undulations, have generally made 

 use of the spectrum formed by refraction. But the figure of the 

 spectrum obtained by refraction is deformed. The more re- 

 fractive parts are elongated, the less refractive shortened ; and 

 it is difficult to ascertain the properties of the component parts 

 of a natural ray of light in this manner. 



Newton, who first endeavoured to express the length of the 

 portions belonging to the seven distinguishable colours of the 

 spectrum, observed an analogy between the lengths of these por- 

 tions and the differences in the numbers given by the values of 

 the tones in an octave of the minor mode. This analogy however 

 is purely accidental; the res[)ective lengths of the different 

 coloured parts of the spectrum formed by refraction vary accor- 

 ding to the nature of the body which is made use of. From 

 the very supjjosition that the spectra foiinefl by ditffront sub- 

 stances are similar to each other, Newton drew tlic erroneous 



2 G 2 



