283 JOURNAL, BOMBAY NATURAL HISTORY SOCIETY, Vol, VIII. 
in tint with variation in the light source is an essential difference, it is not the 
only difference, even in the colour manifestations of the two groups, for it is 
also characteristic of the nature of the colour-producing structure. It is to 
the above-mentioned varying colours that we apply the term “ iridescent,” from 
the resemblance they have in the sequence or play of colours to the tints of 
the rainbow. The unvarying group of colours, having no equivalent term to 
“ iridescence” to express the nature of their colour production, are spoken of 
as “pigmentary,” or absorption colours. In naming examples of objects, 
natural and artificial, grouped as above in accordance with the nature of their 
colours, it is difficult to make a selection where all are so varied and charac- 
teristic. I have preferred, therefore, to cite only such instances as I myself 
possess, and am, therefore, able to show you. As examples of pigmentary 
colours, I need only name one or two for the sake of comparison, since the 
eolours of most objects ordinarily met with are pigmentary. Leaves, flowers, 
dyes, birds, fish, insects, minerals, &c., exhibit these colours, some almost 
entirely, and all, excepting fish, in far the majority of instances. Of objects 
displaying iridescent colours we have also examples in the various divisions of 
the animal, vegetable, and mineral kingdoms. Amongst birds the most striking 
examples are found amongst the Humming Birds, Sun Birds, Birds of Para- 
dise, &c. Insects, again, furnish numerous examples, more especially 
amongst tropical species, though not, perhaps, proportionally in greater 
numbers than amongst those belonging to our own more temperate regions. 
The colours of fish are almost entirely iridescent, since their very whiteness, or 
silvery sheen, is due to the admixture of the iridescent colours of innumerable 
minute thin lamelle, too small to be seen individually with the naked eye, but 
plainly perceptible under the microscope. In the vegetable kingdom irides- 
cent colours are far more numerous than is ordinarily recognized, since the 
surfaces of the cell-walls produce interference colours which are more or less 
obscured by the pigmentary colours of leaves and coloured flowers, but may 
be readily seen in the case of white flowers by the aid of a lens and sunlight. 
Under these conditions each cell may be seen to sparkle with its own iridescent 
colour, forming, by admixture of the interference tints of neighbouring cells, 
the varying shades of white seen in numerous flowers which are devoid of 
pigmentary colour, Mineral bodies displaying iridescent colours are also 
numerous ; opals, sunstone, fire-marble, felspar, mica films, tarnish on various 
metallic crystals, certain crystals of chlorate of potash, &c., are examples. 
In describing the various natural objects for purposes of identification, or 
mere description, no account can be considered complete which omits all re- 
ference to their colours, and more especially is this the case where the colours 
constitute such a striking feature, asin the case of iridescent bodies. In in- 
numerable instances, more especially amongst birds and insects, their specific 
names are taken from some conspicuous colour they possess. It thus becomes 
evident that a correct description of the colours of bodies is of importance, 
