492 BARON VON WREDE ON THE ABSORPTION OF LIGHT 



irregular spectrum, which contains all colours. But in this spectrum 

 we find a perfectly distinct orange-coloured image of the whole flame 

 of light produced from the homogeneous light in the outer flame. If 

 we move the image of the light so that only the inferior blue part 

 may fall on the aperture, we find that the spectrum contains only violet, 

 blue, and green light; but at the same time we find three quite appa- 

 rent and regularly placed maxima, to explain which Me must presuppose 

 a retardation of about 10 or 12 lengths of the waves of light. 



One of the most peculiar of this kind of spectra is, without doubt, 

 that from the light of burning spirits of wine, in which chloride of cop- 

 per has been dissolved. This spectrum is filled with bright stripes, 

 which are so arranged that they always appear in pairs with a black 

 stripe between them, while those belonging to the diiferent pairs are 

 separated by a broader stripe, as it is represented by K L, fig. 5. 

 In order to explain this phaenomenon we only need suppose two re- 

 tardations, one twice as great as the other, and having such a position 

 that the maxima of the smaller one fall on the minima of the greater 

 one. In the first the curve of intensity obtains the form A B, in the 

 latter the form C D ; the resultants of both must consequently have 

 the form E F. If G H expresses the limit for the power of per- 

 ception of the eye, it is evident that the spectrum must receive the ap- 

 pearance of K L. On the contrary, let us imagine that the maxima of 

 both components fall together, so that the one has the position A B 

 (fig. 6), and tiie other that of C D, the resultant then takes the form 

 E F. If now G H represents again the limit of the power of percep- 

 tion of the eye, it is clear that the spectrum produced must contain 

 black stripes, appearing in pairs, separated by bright ones, or must ap- 

 l-'ear as K L (fig. 6)*. 



In the same manner as we can produce with owe piece of mica the phas- 

 uoniena of absorption originating from one I'etardation, just so we can 



* If we put It' =3 hh in the formula (8), it is evident that the maxima or 

 ininima corresponding to W must come to lie where the maxima of b are. The 

 lirst-mentioned case, or tlie appearance in pairs of bright stripes, cannot there- 

 fore take place when one retardation is exactly twice as great as the other ; 

 in such a case it is more likely that the stripes occtirring in pairs would appear 

 black. It is, however, evident that we only need increase the greater retarda- 

 tion by one single wave-length in order to make one maximum which corre- 

 sponds to the retardation fall on a minimum. 



The other maxima and minima do not indeed then completely coincide one 

 with another, but they come evidently nearer to one another the greater the 

 retardations are ; and when these are somewhat considerable, the irregularities 

 arising are so small that the eye is no longer able to discover them. We must 

 suppose in the flame of chloride of copper that the smaller retardation amounts 

 to about 40 wavi -lengths of the red light, i.e. about GO of the violet; the 

 greater need only be increased by eV to ,io above the double value of this 

 majnitude. 



