PRODUCED BY ELECTRO-CHEMICAL ACTION. 103 
variations of the law of refraction, when the obliquities of inclination 
are great,—such, for example, as those to which we must have recourse 
in order to account for the changes of tone in the colours of the first 
two rings. 
Exceptions to the Law of Varying Colours. 
If bodies where composed of thin layers suchas those which form the 
chromatic scale and Newton's rings, their colours would change with 
every change of incidence, conformably to the law which we have just 
indicated. In nature the number of those colours that change is but 
small in comparison with those that remain fixed. Hence it may be 
inferred, either that the colours of bodies depend in general on a princi- 
ple different from that of the colours of thin plates, or that this principle 
is modified in its application, the bodies not being constituted exactly 
as such an explanation would require. A few observations will perhaps 
be sufficient to fix our ideas on this very interesting point in the theory 
of colours. 
Varying Colours in Nature. 
In each of the three kingdoms of nature we have specimens of these 
colours. The animal kingdom especially affords some that are highly 
interesting, both in respect to their number and their beauty. It will 
be sufficient to mention the wings of butterflies and insects, and above 
all the feathers of different birds. Who is there that does not know, 
for instance, the variety of pleasing hues displayed in the plumage of 
the peacock? In this case, as well as in others of a similar kind, the 
colour that we observe is not given out by one continuous surface, such 
as that of a single plate: it is produced by a multitude of threads or 
fibres, so nicely overlapping one another that they seem to form a perfect 
plane, although they are really but a vast number of distinct minute sur- 
faces, the position and thickness of which it would be necessary to know 
in order to apply the general law to them with any prospect of success. 
The phznomenon possesses all the characteristics of that produced by 
thin plates; but instead of a single layer, the number in this case is 
infinite, and, though disposed in an order calculated to excite our ad- 
miration, still it complicates the action of the light so as to prevent us 
from tracing it through all its variations. 
The varying hue most frequently exhibited by the plumage of birds, 
is a beautiful green of the same intensity as No. 32. This number in 
the scale retains almost all its intensity, even at an inclination of 40°: 
at dn angle of 50° it presents the appearance of No. 31, which is a purple 
colour with a greenish tinge ; beyond that the original colour completely 
vanishes, and in its stead we have the violet-lake of No. 30. 
But the varying green of feathers begins to change much sooner : 
when the inclination is near the 40th degree, it already presents the 
