RRPORT ON PHYSICAL OPTICS. 3,^1 



restraint, like a string which has been stopped and detained in 

 any part of its vibration on either side of the centre. 



The fixed lines in the solar spectrum first noticed by Wollaston, 

 and afterwards more minutely traced by Fraunhofer, have lately 

 been examined with great care, and with his usual success, by 

 Sir David Brewster ; and he has observed a remarkable coinci- 

 dence between these lines and the dark bands of the spectrum of 

 the nitrous acid gas *. Sir David Brewster has also studied, in 

 connexion with the same subject, the definite absorbing effects 

 of the earth's atmosphere. This has been effected by examining 

 the solar spectrum, when the sun was near the horizon ; and it 

 has been found that most of the dark bands thus developed be- 

 longed to the fixed lines of Fraunhofer, which were thus, as it 

 were, widened, and brought out, by the absorptive action of the 

 atmosphere. A similar result has been arrived at in other cases, 

 and it has been found that the points of the spectrum on which 

 absorbing bodies exert the strongest specific actions are gene- 

 rally coincident with the deficient rays of solar light f. This 

 singular connexion gives considerable weight to the speculations 

 of Sir David Brewster respecting the latter phenomena |. 



The observation of the fixed lines in the solar spectrum led 

 Fraunhofer to examine the optical characters of the lights ema- 

 nating from other sources. He thus arrived at the interesting 

 discovery, that the system of bands in the different species of 

 light which he examined, varied with the source ; while it was 

 constantly the same m the number of the bands, and their 

 relation to the coloured spaces, in the light of the same 

 source, however modified. In the light of Sirius there are three 

 broad bands which have no resemblance to those of solar light. 

 The light of the electric spark, on the other hand, when ana- 

 lysed by the prism is found to have several bright lines, of 

 which that in the green is remarkably brilliant. Similar plie- 

 nomena were observed in the light of artificial flames, — the flame 

 of an oil lamp, for example, exhibiting a well-defined bright 

 band between the red and yellow, and another not so distinct 

 in the green §. This however is not universally the case. In 

 the red flame of strontia, as was observed by Dr. Faraday and 

 Mr. Talbot, there are a number of red rays separated from each 

 other by dark bands ; and in the flame of cyanogen, when 

 similarly analysed, the violet is found to be divided into three 

 distinct portions with broad dark intervals §. 



* " On the Lines of the Solar Spectrum," EJin. Trans., vol. xii. 

 t " On the Colours of Natural Bodies," Ed'm. Trans., vol. xii. 

 X Report on Optics. § Munich Memoirs. 



II Phil. Mag., Third Series, vol. iv. p. 114. 

 1834. Y 



