The Structure of Coloured Bodies. 197 



feathers of many humming-birds, e.g., tail feathers of 

 the blue-tailed sylph, Cyanthus forficatus, the glossy blue 

 black plumage of many tropical and British birds, the 

 purplish blue patch (speculum) in the wing of the mallard, 

 innumerable flies, dragon flies, beetles, and butterflies, 

 and many minerals, as labradorite, opal, &c., mother of 

 pearl, and also bodies artificially prepared, as thin films 

 ■of mica, certain crystals (twin crystals) of chlorate 

 ■of potash. It is needless here to individually specify these 

 various objects, because in their appropriate sections 

 I purpose mentioning striking and typical examples from 

 the various groups of coloured bodies for the purpose 

 of drawing attention to the nature and properties of their 

 colours and the structures producing them. The above 

 sequence of colour phenomena is what is commonly observed 

 in almost all iridescent natural bodies in which the colours 

 are due to thin plates. How constant this change of colour is 

 maybe inferred from the fact that, keeping in mind thesimple 

 principle which governs the production of colour by thin 

 plates, I was able to predict, without a single mistake, 

 the sequence of changing tints that arose by regarding from 

 different points of view the numerous specimens constituting 

 that magnificent collection of humming-birds known as the 

 Gould Collection at the South Kensington Museum. 



It was this constancy in the colour phenomena pre- 

 sented by thin plates that naturally suggested the inference 

 that if such colour phenomena zvere really constant and peculiar 

 to thin plates, sue J L appearances might be accepted as proof of 

 the existence of a structure of thin plates, even though such 

 structure might not be apparent by ordinary microscopic 

 investigation alone. But the fact that the same colours and 

 sequence of colours are observed on viewing a structure 

 composed of a series of fine lines, at once proves that 

 such colours, so far as regards their composition 

 or tint, and also as regards their sequence, are identical : 



