418 DR. He GADOW ON THE COLOUR OF FEATHERS. [May 2, 
red lying at 7 (2. e. towards the upright edge of the prism), and with 
the violet at v. 
No rays, or at least no visible ones, are 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 facts 
Fig. 2. 
Al 
XX XK 
= exn®* = 
= 2 
& * 
~ * 
Wee 
* 
PN 
* 3 
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? 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 
of the spectrum { may be concealed, in which case that feather 
would only vary between red and greenish. Or two neighbouring 
prisms, even if they belong to the same radius or barbule, may be so 
situated that their spectra partly overlap one another. This would 
have a double result: first, that where two complimentary colours fall 
upon each other they would simply produce white light; secondly, 
