Iridescent Colours. 149 



On Iridescent Colours and a Method of examining 

 Iridescent objects, Birds, Insects, Minerals, &c, so 

 as to ensure uniformity in their description. By 

 Alex. Hodgkinson, M.B., B.Sc. 



{Received March 1st, 1892.) 



On taking a general survey of coloured objects, whether 

 natural or artificial, we become aware of the fact that whilst 

 the colours of some remain unchanged as regards tint, 

 whatever their position in relation to the incident light, the 

 tint of others varies with every alteration in their relation- 

 ship to such light source. We thus see that so far as their 

 colours are concerned all bodies may be arranged in two 

 groups according as their colours change or do not change 

 in tint as their angular relationship to the light varies. 

 Nor is this classification entirely an artificial one, since, as 

 will shortly be seen, though this change 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 

 characteristic. I have preferred therefore to cite only 

 L 



