Page:Scientific Memoirs, Vol. 1 (1837).djvu/115

Rh 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 such as 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 principle 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 surfaces, 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 phænomenon 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 admiration, 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 an 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