Permanent Colours of Opake Bodies. 1 1 1 



Before this fuperficial covering is attenuated, 

 by the heat, it is much denfcr, fo that, in fome 

 parts of the fhells, it appears black : as it is too 

 denfe to admit the paflage of the light to the 

 white fubftance of the fliell, and back again. But 

 where this tranfparent blue colour is fpread thin- 

 ner, and rarer, the light, reflefled from the white 

 fubftance of the fhell, is tranfmitted back through 

 the blue film, and yields alight blue colour. 



The colour, of the fhells of feveral fpecics of 

 eggs, is alfo merely fuperficial, and may be fcrap- 

 ed off, leaving the white earth of the (hell ex- 

 pofed to view. 



Feathers, in like manner, owe their colours to 

 thin layers of Coloured Matter, covering the white 

 fubftance, of which they are principally formed." 

 I fcraped off the fuperficial colour from fuch parts 

 of vividly coloured feathers, as were folid enough 

 to admit of that operation, and, by this means 

 feparated the coloured layers from the white 

 ground, on which they had been naturally fpread. 

 The furfaces of the lateral fibres of feathers 

 cannot be thus feparated on account of their mi^ 

 nutenefs. But, as they appear, when viewed in a 

 microfcope, nearly to refemble, in their form, the 

 feathers themfelves, it feems probable that their 

 colours arife from a fimilar matter, and confor- 

 mation, in the fmaller fibres, as in the grofler 

 parts of the feathers. 



The colours of all the animal fubftances, which 

 have fallen under my obfervation, are effefted in 



P 2 the 



