524 Sir DAVID BREWSTER on the Lines of the Solar Spectrum, 



From all these difficulties, the discovery of lines in the nitrous 

 acid gas spectrum completely relieves us. As the lines whose 

 distances are required, may be made as broad and black as we 

 please, prisms of ordinary purity are sufficient to exhibit them in 

 perfect distinctness. The artificial light of a lamp can be com- 

 manded at any hour, and as its rays are absolutely fixed, the least 

 experienced observer can have no difficulty in measuring the dis- 

 tances of the fixed lines, and thus obtaining, with extreme ac- 

 curacy, all the data for the construction of achromatic instru- 

 ments. 



But it is not merely to this practical purpose that the ga- 

 seous lines are singularly applicable. Among the various solids 

 and fluids in nature, there are very few sufficiently pure and 

 transparent, to enable us to see through them the lines of the 

 solar spectrum, so as to enable us to measure their refractive 

 and dispersive powers with minute accuracy, whereas the gaseous 

 lines can be rendered visible, however imperfectly the spectrum 

 may be formed. In determining the various elements of double 

 refraction and polarisation, and in all optical researches where 

 the phenomena vary with the refrangibility of the rays, the ga- 

 seous lines will hereafter perform a most important part. 



Had the solar lines been much broader than they are, we might 

 have been able, by means of minute thermometers, to have ascer- 

 tained the temperature of all those parts of the spectrum where 

 there was no light, and thus to have determined whether or not 

 the rays of light and heat are separate and independent emana- 

 tions. The phenomena of the nitrous acid gas spectrum, the lines 

 of which can be widened at pleasure, enable us to perform this 

 and other interesting experiments, and thus to decide many im- 

 portant questions in the theory of radiant matter. 



From the various experiments which I had made on the ab- 

 sorptive action of coloured media, I was led to a general prin- 

 ciple, which, in that stage of the inquiry, appeared to possess con- 

 siderable importance. The points of maximum absorption exhi- 



