7, 
7 
12 M. Melloni on the Radiations of Incandescent Bodies, 
On observing attentively the rectangular form of the space oc- 
eupied by the yellow rays in each of these spectra, it will 
distinctly perceived to be less elongated in the elementary spec- 
trum than in the compound spectrum. Now distinct vision may 
diminish the magnitude of the image formed upon the retina, and 
render the contours more decided and sharp, but it cannot vary 
the relations of its dimensions. The shorter length of the yel- 
low rectangle in the elementary spectrum must then arise from 
an effect different from that which is produced on vision by the 
smallness of the aperture through which the prismatic image Is 
observed. We cannot, therefore, suppose, the vertical sides of 
the rectangle being a little sharper and more vivid than the hori- 
zontal ones, that this difference of illumination can give origin to 
the phenomenon in question ;—for any alteration arising in that 
way would be in the opposite direction to that actually observed. 
For, in reality, the most luminous images being those which 
undergo the greater reduction of size in passing from confused 
to distinct vision, the yellow fury eria of the spectrum observed 
in the elemer m ought to be shorter in the horizontal than 
in the vertical “direction ; nevertheless, the difference is manifest- 
ed in just the opposite manner. Therefore, the elongation of 
the yellow rectangle in the central spectrum arises beyond all 
doubt, altogether or in part, from a partial a of the 
yellow rays belonging to the spectra of the entir of ele 
mentary prisms, the rays from which traverse oe dilated pupil 
of the observer. This overlapping takes place perpendicularly to 
the axis of the spectrum, and consequently in a vertical direction 
in the arrangement adopted in my experimen 
he method proposed by Sir D. Brewster to determine the 
me adapted to the end in view. And, until it shall have ai 
proved, that the colors of a spectrum, perfecth y pure, —— by 
the interposition of a medium placed near the prism, the change 
persisting at every other distance, the existence of disockaaal 
in the same transverse element of the spectrum ought to be re- 
garded as entirely hypothetica. 
I can therefore no longer admit the existence of different col 
ors in the same part of the spectrum; I hold it as proved, that 
every particular tint arises from a single ray possessing a partic 
lar frequency of vibration, anda particular wave-length. I chieik 
_ altogether with Nospenn that color is a characteristic sign; dis- 
‘tinetive of the different elements contained in the beams of the 
‘sun and of luminous bodies; elements which are separated 
aly under the action ‘of the prism in virtue of their dit 
ees of refrangibility. 
FSi Rr te. 
ng a means of analysis which has enjoyed un a 
favor among i we eagles pbeagiees i 
