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Show 418 DR. H. GADOW ON THE COLOUR OF FEATHERS. [May 2, red lying at r (i. e. towards the upright edge of the prism), and with the violet at v. No rays, or at least no visible ones, arc thrown out into the space between 1 and 2 ; consequently with our eye at 1, corresponding to position A, we shall perceive black. At 3 the first red rays will become visible, at 4 the blue ones, and so forth, till at 6 we come across the ultra-violet rays, where we see again black, corresponding with position C. Between 1 and 6 will be a place from where we can look at the object under full light (position B ) ; and this, of course, is the way in which we generally describe an object. As this agrees with observation, i. e. as every metallic feather (if examined in the way explained above) shows precisely the same phenomena as a prism under similar circumstances would show, we have every right to consider the explanation of " metallic varying colours" as proved. There are, however, several observed fact's Fig. 2. S < ^ * -*-0 S Diagrammatic section through the barb of a " metallic " feather. which need an explanation, since they seem rather to upset this theory. First, why does not every metallic feather display all the colours of the spectrum ? and why do they generally range not over more than a few neighbouring colours 1 Of course any prism, however small it be, displays all the colours of the spectrum; but this does not mean that all of them reach our eye! Part of the spectrum might be hidden by some other object standing between it and our eye; for instance we can easily cut off either end of a spectrum by a screen. In the feathers the screen would be represented by a neighbouring radius, so that, as, for instance, in fig. 2, the lower half ot the spectrum ft may be concealed, in which case that feather would only vary between red and greenish. Or two neighbouring prisms even it they belong to the same radius or barbule, may be so situated that their spectra partly overlap one another. This would upon LS reH,t: ""'I ihat WlTe ^° c™Pl*™ntary colours "all upon each other they would simply produce white light; secondly, |