HUMMING-BIRDS. 
115 
quently suspend themselves by the feet, with their 
heads downwards, in the manner of some parrots.” 
The structure of the feathers, which shine with so 
much lustre, has occupied the attention of most of 
their describers. Audebert has tried to demonstrate 
the cause on mathematical principles, the form of the 
feathers, and the manner in which the light strikes 
them ; while Lesson is of opinion, that the colours 
are due to elements contained in the blood, and dif- 
fused by circulation. He says, at the same time, 
that all the barbules and plumulets are deeply fur- 
rowed in the centre, and the light, when striking ver- 
tically, produces no colour, or only black ; but when 
striking transversely, every opposite side of the furrow 
acts as a reflector to the others, and in this way assists 
in producing the colours. 
Bullock, when speaking of the same subject, says, 
that “ the preserved specimens were but the shadow 
in brilliancy to what they were in life. The reason is 
obvious ; for the sides of the laminee, or fibres of each 
feather, being of a different colour from the surface, 
will change when seen in a front or oblique direction ; 
and as each lamina or fibre turns upon the axis of the 
quill, the least motion, when living, causes the fea- 
thers to change suddenly to the most opposite hues.” 
We have thought it proper to mention those different 
opinions ; and though they do not entirely coincide 
with our own, we are not at present able to explain 
all the causes. In birds possessing this shining and 
metallic variation of lustre, we have found the struc- 
