IRIDESCENT COLOURS. 284 
and where these colours are of the pigmentary or unchanging kind, this is a 
matter of no difficulty. How different, however, in the case of objects, the 
colours of which not only vary with every change of position, but disappear 
altogether, unless viewed with special relation to the light source, Nor can 
it be wondered at that descriptions of these objects, even by observers of 
undoubted repute, vary according to the different angles from which they 
have been viewed, or are vague and profuse, owing to fruitless attempts to 
describe their changing tints produced by every movement, The fact is, no 
words can convey an adequate impression of the gorgeous effects produced by 
most of such objects, whether birds, insects, or fish, when in motion in bril- 
liant sunshine. Some notion of the difficulties to contend with in describing 
the colours of humming birds, for example, may be gathered from the 
remarks of Wallace in his work on “ Tropical Nature,” when speaking of 
humming birds :—“ In some species they must be looked at from above, in 
others from below ; in some from the front, in others from behind, in order to 
catch the full glow of the metallic lustre ; hence, when the birds are seen in 
their native haunts, the colours come and go and change with their motion, so 
as to produce a startling and beautiful effect.” Most observers, in describing 
the colours of iridescent bodies, do so by attempting to depict the varied 
effects produced by casually changing the position of the object in relation to 
the light, omitting to mention the exact sequence of the play of colours, or the 
relation of these colours to the direction of the iridescent light, i.e., whethe 
produced by perpendicular or oblique illumination, Here is a description of 
the tufted neck humming bird, Zrochilus ornatus, taken haphazard from a 
well-known work :—‘‘ The throat is of a fine green colour, variable in differ- 
ent lights to a golden hue with a yellow or brown metallic lustre, and below 
that the whole of the belly is a rich brown, glossed with green, and golden.” 
Such descriptions as the above, which happen to be the first I met with in 
seeking for an instance, are vague, and fail to give a definite idea of the 
appearance of the object. But vagueness in the description of these objects 
is not the only result of the changing character of their colours, As might be 
expected, where such variation in appearance exists, the descriptions of differ- 
ent authors are almost as variable as the colours, Few attempt descriptions 
without acknowledging the hopelessness of the task, Thus J ardine, after 
describing this humming bird, Chryslampis mosquitus, remarks : “It ig impos- 
sible to convey by words the idea of these tints, and having mentioned those 
substances to which they approach nearest, imagination must be left to con- 
ceive the rest,” And I adduce this quotation as fairly expressing the feeling 
of naturalists in reference to the description of iridescent objects general- 
ly. Recognizing the admitted inability of observers to convey by description 
an idea of the appearance of these iridescent objects, and having myself, for 
many years, constantly experienced the same difficulty, I have been led to 
adopt a method for the examination of such objects, which, whilst extremely 
