ACCORDING TO THE UNDULATORY THEORY. ^QI 



der G H as the limit of perceptibility for the eye, we obtain a spectrum 

 exactly like that above described. 



The phaenomena which appear in the spectrum of coloured flames 

 stand undoubtedly in connection with the present object, and may be 

 explained iu the same manner as the phEenomeua of absorption. But 

 besides the presupposition of retardations, we must here still make an- 

 other, namely, that certain flames can only produce light of a certain 

 length of wave, or at least that the produced light is inclosed in certain 

 limits, which lie closer to one another than the red and violet. Various 

 phaenomena in the spectra of coloured flames arise only from this 

 cause. This, for example, is the case with the bright orange-coloured 

 stripe which is formed in the spectrum of a comm<m candle light. If 

 we consider the light of a candle, we find it to consist of several divi- 

 sions, differing from one another. The inner and lighting flame con- 

 tains, as is well known, heated particles, which undergo a real combus- 

 tion then only when they arrive at the outermost border, or where they 

 come into contact with the air. The outer flame is therefore of quite 

 a different consistence from the inner; it has also quite a different 

 appearance ; it lights feebly, and possesses a faint orange colour. The 

 broadest part of the flame has, on the contrary, a blue colour, and re- 

 sembles in every respect that produced by a slow combustion of coals. 

 As this flame originates at the point where even the wick comes into 

 contact with the air, I consider it quite probable that it arises from a 

 slower combustion of it. 



If we place a convex lens between the flame and the opening through 

 which we allow the light to fall on the prism, so as to produce a mag- 

 nified image of flame on the prism, we are then able to bring to the 

 opening any part of the image of the flame by sliding the lens, and 

 in that manner to examine any particular part of it. Suppose we then 

 move the image so that only its outermost border lies on the aperture, 

 and consequently only the light of the most external flame can pass 

 through it, we find that the spectrum contains nothing else than a part 

 of the orange- coloured stripe. If we change the form or breadth of 

 the aperture, we find that the orange-coloured stripe undergoes just 

 the same change, so that it always remains a complete copy of the aper- 

 ture. If we slide the image of the flame so that the inner lighting part 

 arrives at the aperture, we obtain a complete spectrum ; and the nearer 

 the middle part of the flame comes to the aperture the greater is the 

 briglitness the spectrum acquires, while the orange-coloured stripe de- 

 creases more and more. Hence I conclude that the inner flame pro- 

 duces light of all possible lengths of wave ; the outer flame, on the con- 

 trary, only light of a single length of wave, that is, of a completely Jio- 

 mogeueous light. If we view a flame of light through a prism, without 

 letting the ligiit pass through a minute aperture, wc naturally obtain an 



