The Structure of Coloured Bodies. 201 



■angle equal to that of incidence, a feature which distinguishes 

 it at once from a structure of fine Knes. 



We see from these experiments that, however closely 

 the colour manifestations of these two colour-producing 

 structures may resemble each other to ordinary observation, 

 when in the above typical form, and submitted to some such 

 method of examination as the above, the resemblance breaks 

 ■down, and we have presented phenomena so markedly dis- 

 tinct as scarcely to admit of confusion. Could we, therefore, 

 ensure these conditions being always complied with, our 

 investigation would be a comparatively simple matter. 

 Such is, of course, not the case. Whilst our method of exami- 

 nation, being adapted for opaque as well as transparent 

 objects, is constantly applicable, infinite variations from the 

 typical condition of the structures examined exist. In the 

 case of thin-plate structures we shall find them wonderfully 

 constant in their manifestations, but even here I shall draw 

 attention to natural and artificial bodies in which colour 

 phenomena of singular interest and beauty are produced by 

 the superposition of numerous iridescent plates, seen in the 

 case of most iridescent beetles and flies, silvery-scaled 

 fish, and certain twin crystals of chlorate of potash, &c. 

 When other methods of colour production co-exist with that 

 of thin plates, as we find of constant occurrence, the appear- 

 ances, though more complex, still admit of analysis if 

 properly examined, and, each phenomenon having its own 

 -Structural significance, the results of examination in such 

 instances are of more than ordinary interest. Thus, in the 

 case of mother of pearl zcheji groiDid, we have an example 

 of a structure composed of thin plates and fine lines, and, 

 accordingly, this substance yields colour phenomena bearing 

 the characteristics of both these structures. As shown by 

 Brewster, the diffracting structure of this substance is 

 communicable to wax. I shall show that in the case of 

 mother of pearl, however, the diffracting structure is 



