288 THE ZOOLOGIST. 
Fatio, in his interesting and suggestive paper on ‘‘ Des Diverses 
Modifications dans les Formes et la Coloration des Plumes” 
(‘ Mémoires’ Soc. Phys. et H. N. de Genéve, xviii. 1866), quotes 
briefly other Continental authors who have held this theory of 
thin plate colours. Mr. C. HE. Benham has been good enough to 
put me on the track of work by the late Mr. Lewis Wright, an 
interesting writer on the science of colour and light, and far 
better known as an expert on the economical aspects of the 
Common Fowl; but, although I have not been successful in 
seeing any long study of metallic feathers, he was clearly a 
believer in the colour being due to thin plates. 
Many insects and molluscs exhibit interference colours. The 
shell of Haliotis gives wonderful effects, and responds to light in 
exactly the same way as does the plumage of the Humming Bird 
(see Dr. Gadow’s diagram of ‘‘ Positions for viewing Prismatic 
Feathers,’ P. Z.8., 1882). In the case of the shell it is easy to 
demonstrate, by the simple test of using alternately transmitted 
and reflected light on a minute flake under the microscope, that 
the colours are really due to thin plates and not to prisms. 
Quite recently (cf. ‘Nature,’ May 17th, 1912, p. 272) it has been 
proved that the iridescent colours of insects are due beyond any 
question to thin plates, and may be altered by the application 
of pressure. 
Perhaps I might digress a moment to state that “ thin 
plate’ colours are produced by the action of thin films of any 
transparent substances on ordinary light. Speaking roughly, 
we only get these colours when the thickness of the film does 
not exceed ‘000015 of an inch or thereabouts, and every altera- 
tion in thickness alters the colour of the light reflected or trans- 
mitted. A simple experiment is to drop a minute quantity of 
turpentine on the surface of warm water, and then observe the 
changes of tint as the film spreads and evaporates. When we 
hold the water in a shallow black japanned tray the effects are 
wonderfully brilliant, and everybody must have noticed the 
vivid colours even on patches of oil in wet streets. It is wrong 
to call these colours ‘‘ prismatic.” They have nothing whatever 
to do with prisms, and, moreover, we get tints that are unknown 
in the spectrum. As arule, the brilliant colours alone catch the 
ordinary eye, but a moment’s attention will show the presence 
