er 
be 
& 
M. Melloni on the Radiations of Incandescent Bodies, 
“ As (in the experiment in which a metal is ignited ),” adds Pro- 
vibratory movement executed by the molecules of the platinum, 
it seems from the foregoing considerations to follow, that the fre- 
quency of those vibrations increases with the temperature.* In 
this observation I am led by the principle, that ‘to a particular 
color there ever belongs a particular wave-length, and to a par- 
ticular wave-length there ever belongs a particular color ;’ but in 
the analysis of the spectrum made by Sir D. Brewster by the aid 
of absorptive media, this principle is indirectly controverted ; that 
eminent philosopher shewing that red, yellow, blue, and conse- 
quently 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 dis- 
persion, will paint these several spectra on the screen ; the colors 
of the one not coinciding with, but overlapping the colors of the 
other. In such a spectrum there must undoubtedly be a general 
commixture of the rays; but may we not fairly enquire whether, 
if an elementary prism were used, the same facts would hold 
good ; or, if the ws et face of the prism were covered by a 
expose a w fissure parallel to the axis of the 
instrument, would there ines found § in the spectrum it gave ever 
color in every part, as in Sir D. Brewster’s original experiment ? 
M. Melloni has shewn how this very consideration complicates 
the phenomena of radiant heat; and it would seem a very plau- 
sible suggestion that the effect here pointed out must occur in an 
analogous manner for the phenomena of light.” 
I shall here observe frankly, that the optical complication (al- 
luded to in this criticism) does take place, as may be clearly prov- 
ed; it is this very circumstance which led me to detect the errors 
which had been committed in the determination of the maximum 
‘of temperature in spectra arising from different substances. That 
* This sxpreerens ought not to be taken in an absolute sense, but rather as rela- 
tive to the new rays which at a given hn are added to those already ek 
isting in the spectrum.—(.Note 
+ Sir pe vid Brewster has replied to Pookeee Draper as follows :—* As m 
perim B epecs formed upon screens by prisms sit. fons 
refracting surfaces, they ae nat able to this criticism, even if it were otherwise 
well fo The spectra which I use are so pure, and free from all commix- 
be that va raunhofer’s black lines are distinctly visible ; and the results are precisel 
the same when the refracting face of the prism is reduced to the smallest pom 
ray al of the spectrum by absorption, does not therefore ris = 
t the principle, that ‘to a particular color there ever belon riicular 
to a par rticular eae length there ever belongs a partion| ar col: vr, 
tes, in theoreti cal language, the well known oben of Sir 
aoe it ditettly controverts it, and absolutel —Philos. 
—(Note of M. de la R.) 
