286 THE ZOO0OLOGIST. 
seen in any feather, and upon a complete reversal of the laws 
governing the action of a prism upon a beam of light. This 
theory, which originated more than half a century ago with 
Prof. B. Altum (‘ Naumannia,’ 1854, pp. 293-804; and ‘ Journal 
fur Ornithologie,’ 1854, p. xix.), suggests that the colours of 
metallic feathers are due to the action of a series of prisms in 
the structure of the feather; but his papers were not based 
upon any rigorous microscopical examination, and his views 
could never have attained their present currency in England 
had they not been repeated in 1882 by Dr. H. Gadow (P.Z.&., 
1882), who was cautious enough to add (p. 418): ‘‘ There are, 
however, several observed facts which need an explanation, since 
they seem rather to upset this theory.” Unfortunately, this 
caution was not extended to the more widely circulated article 
in Newton’s ‘ Dictionary of Birds,’ which is marred by several 
faults—obvious enough to the thinking reader or the original 
observer, but likely to be overlooked by the general ornithologist. 
In the earlier paper a diagram of a “prismatic” feather is 
properly termed ‘‘ hypothetical,” but in the ‘ Dictionary’ article 
the figure is reproduced above the inexcusable title, ‘‘ Dia- 
erammatic Section through the Barb of a Metallic Feather.” 
I have examined some hundreds of sections of metallic 
feathers under the microscope, and have never seen anything 
even remotely resembling this figure; and it should be obvious 
that, even if these prismatic layers did exist, their spectra would 
be projected inwards and so be quite invisible. Too much stress 
is laid on the assumption that these metallic colours are always 
those of the spectrum; yet in the eyed feather of a Peacock’s 
train we can easily isolate sage green, raw sienna, burnt sienna 
(I use these well-known names advisedly), and other impure 
tints. It will thus be seen that the objections to the “‘ prismatic 
theory’ are very great: (1) the prisms have never been seen; 
(2) if they did exist, their spectra, according to the laws 
governing the path of light through a prism, would be invisible ; 
and (3) other colours besides pure prismatic colours are con- 
spicuous on many metallic feathers. 
The true explanation is that the colours are thin plate or 
interference colours. This is no new discovery, and the fact 
seems to have been accepted without question by physicists for 
