1 873.] Colours and their Relations. 87 



light of one pure colour is thrown on the lenses, the rings 

 are all of that one colour, and merely light and dark — the 

 waves alternately doubling and extinguishing the effects of 

 each other. Their breadth is greatest with red and least 

 with violet light; while it is by the overlapping of these 

 rings and the consequent intermingling of their tints that 

 the succession of colours is produced when white light is 

 employed. 



Another method of exhibiting these beautiful rings is by 

 blowing soap bubbles of a large size. This may be done 

 by using a mixture of soap and glycerine, and the bubbles 

 thus obtained may be preserved for several hours intact 

 under a bell-glass. The colours are here produced by the 

 interference of the light coming from the inner surface of 

 the film with that coming from its outer surface. The two 

 surfaces are most nearly approximated at the summit of the 

 bubble, and they gradually separate thence downwards, so 

 that the same conditions are present as in the case of the 

 two lenses. Another simple way of producing this system 

 of rings is by spreading a thin film of soap over a glass plate, 

 and breathing on it through a finely pointed metal tube. In 

 this case the effect is due to the condensation of the breath 

 into minute hollow vesicles, which increase in size from the 

 centre outwards. They are, in fact, diminutive soap bubbles. 



This class of colours goes under the general denomination 

 of the colours of thin plates, and the colours of many 

 natural objects fall under this category. Among the most 

 beautiful of these, and the most nearly allied to Newton's 

 rings, are the colours exhibited by the discoid frustules of 

 certain of the Diatomaceae. These consist of very thin 

 superimposed plates of pure silica, ornamented with various 

 patterns, produced by extremely minute papillary projections. 



To this same class belong the colours seen in the scum 

 floating on the surface of some liquids, especially of solutions 

 containing salts of iron ; also the colours of fibres and of 

 feathers very generally. The colour of some feathers, how- 

 ever, are intrinsic, consisting of colouring-matter lodged in 

 pigment cells, whence it can be removed and separately ex- 

 amined. The most interesting case of the kind is that of 

 the red feathers in the wings of the plaintain-eater (Musophaga 

 violacea) and the turacu (Turacus albocristatus) , which owe 

 their red colour to a pigment that has been named turacine. 

 This pigment possesses dichroism, being of a deep violet 

 purple by reflected light and crimson by transmitted light. 

 It presents the great peculiarity of containing nearly 6 per 

 cent of metallic copper, which must have entered with the 



