4.92 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 we 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 different pairs are 
separated by a broader stripe, as it is represented by K L, fig. 5. 
In order to explain this phenomenon 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 GH expresses the limit for the power of per- 
ception of the eye, it is evident that the spectrum must receive the ap- 
pearance of KL. 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 the other that of C D, the resultant then takes the form 
EF. If now GH 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- 
pear as K L (fig. 6)*. 
In the same manner as we can produce with one piece of mica the phe- 
nomena of absorption originating from one retardation, just so we can 
* If we put b!= 46 in the formula (8), it is evident that the maxima or 
minima corresponding to 6’ must come to lie where the maxima of 6 are. The 
lirst-mentioned case, or the 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 occurring in pairs would appear 
black. Itis, however, evident that we only need increase the greater retarda- 
tion by one single wave-length in order to make ome 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 wave-lengeths of the red light, i.e. about 60 of the violet; the 
greater need only be increased by ;'5 to ,}. above the double value of this 
magnitude, 
