596 - OPTICAL CHARACTER OF SOME BRILLIANT ANIMAL COLOUES effects. Mr Onslow has shown me many drawings of sections in perpendicular to the surface from many butterfly scales and from 1 three beetle wing-cases. Most of these exhibit structures approxii periodic along the surface, but in no case a structure periodic in inwards along the normal. But a structure of the latter kind ad< to the purpose may probably lie close upon the microscopic limit, i indeed, it could be made evident in a section cut very obliquely. It must be confessed that much still remains to be effected t( a complete demonstration of the origin of these colours. Even if we an interference character, questions arise as to the particular manne there are perhaps possibilities not hitherto contemplated.