and on the Elementary Colours of the Solar Spectrum. 269 



trum. These criticisms acquire more importance since tlie 

 illustrious mathematician, Airy, the Astronomer Royal of 

 England, has denied the deductions of Sir D. Brewster, and 

 refers to several of his experimental researches in support of 

 the view, that a particular colour corresponds to each element 

 of the spectrum*. 



" As (in the experiment in which a metal is ignited)," adds 

 Professor Draper, " the luminous effects are undoubtedly 

 owing to a vibratory movement executed by the molecules of 

 the platinum, it seems from the foregoing considerations to 

 follow, that the frequency of those vibrations increases with 

 the temperaturef. In this observation I am led by the prin- 

 ciple, that 'to a particular colour there ever belongs a parti- 

 cular wave-length, and to a particular wave-length there ever 

 belongs a particular colour;' but in the analysis of the spec- 

 trum made by Sir D. Brewster by the aid of absorptive 

 media, this principle is indirectly controverted; that eminent 

 philosopher showing that red, yellow, blue, and consequently 

 white light, exist in every part of the spectrum. This must 

 necessarily take place when a prism which has a refracting 

 face of considerable magnitude is used; for it is obvious that 

 a ray falling near the edge, and one falling near the back, 

 after dispersion, will paint these several spectra on the screen ; 

 the colours of the one not coinciding with, but overlapping 

 the colours of the other. In such a spectrum there must un- 

 doubtedly be a general commixture of the rays ; but may we 

 not fairly inquire whether, if an elementary prism were used, 

 the same facts would hold good ; or, if the anterior face of the 

 prism were covered by a screen, so as to expose a narrow 

 fissure parallel to the axis of the instrument, would there be 

 found in the spectrum it gave every colour in every part, as in 

 Sir D. Brewster's original experiment? M. Mellon i has 

 shown how this very consideration complicates the phasnomena 

 of radiant heat; and it would seem a very plausible suggestion 

 that the effect here pointed out must occur in an analogous 

 manner for the phaenomena of light J." 



* Phil, Mag., No. 199, Feb. 1847. Sir D. Brewster's refutation of the 

 criticisms of Mr. Airy will be found in our Number for March 1847. — 

 Ed. Phil. Mag. 



f This expression ought not to be taken in an absolute sense, but rather 

 as relative to the new rays wliich at a given temperature are added to those 

 already existing in the spectrum. — (Note of M. iMelloni.) 



J Sir David Brewster lias rci)lied to Professor Draper as follows: — "As 

 my cxperitncnts were not made upon spectra formed upon screens by prisms 

 with large refracting surfaces, they arc not liable to this criticism, even if 

 it were otherwise well-founded. The spectra which I use are so [lure, and 

 free from all conunixture, that Fraunhofcr's black lines are distinctly visible; 



