IRIDESCENT COLOURS. 286 
a perforation in the mirror. When examined in this way, iridescent objects 
are marvellously altered in appearance, their changing colours are replaced by 
one fixed tint, visible only in one position—a fact which serves at once to dis- 
tinguish them from bodies coloured by absorption, which remain coloured, 
whatever the relation to the incident light. Such methods of examining bodies 
scarcely takes more time than by the eye alone. The mirror may be attached 
to a spectacle frame so as to leave both hands free, such as the one I show, or 
may be a simple hand mirror, For objects too small to be seen by the un- 
aided eye, I have so arranged the microscope that light is made to pass down 
the tube of the instrument, through the object glass on to the objects, and by 
a Special arrangement, so adjusted the position of the object that the light is 
reflected back again through the instrument to the eye. The method is thus 
available for macroscopic as well as microscopic objects. 
To illustrate the practical value of this plan of examination, I have here 
a few objects exhibiting iridescent colours, which, by trial, will be found to 
give the following results :— 
The crest of this humming bird, Chrysolampis mosquitus, which, to the 
unaided eye, appears resplendent with all shades of red, orange, yellow, or 
green, according to the angle of the incidence light, appears, when examined by 
the mirror, of one unvarying red tint, disappearing when the object is moved, 
but absolutely unchanging in tint. Such an object, therefore, I should 
describe as “iridescent red,” all else regarding its colour may be inferred. 
Again, the breast, or gorget, of the same bird reflects all shades of orange, 
yellow, or green to the eye alone ; with the mirror it is seen of a deep orange, 
which, as before, is unchanged in tints by any variation in position. Such an 
object I would describe as “ iridescent orange.’ The gorget of another 
humming bird, Calliphlox amethystina, to the eye alone, appears crimson, orange, 
yellow, or green ; with the mirror it is iridescent crimson only, spectroscopically 
a red of the 2nd order. Amongst insects, instances of iridescent species are 
numberless, the results of examination are just the same as in other iridescent 
bodies. ‘This butterfly, Morpho, to the eye alone appears either greenish-blue, 
blue, or violet, as its inclination to the light varies ; examined with the 
mirror it appears green, and should be described as iridescent green, or iride- 
scent bluish-green. This beetle, Poropleura bacca, appears any shade of red, 
yellow, or green to the eye alone ; with the mirror only iridescent red, In 
this extraordinary beetle, Chrysochroa fulminans, we have all the colours of 
the spectrum in their natural sequence, beginning with red at the tip of the 
wing case, and ending with violet higher up the elytron, These colours 
vary in an indescribable manner when attentively examined at different angles 
of incident light with the eye alone ; with the mirror the wing cases are seen 
to be coloured successively from base to tip iridescent green, yellow, orange, 
and red, and these tints remain unaltered by change of position of the object. 
This piece of Haliotis shell exhibits indescribable changes of colour with every 
