The Structure of Coloured Bodies. 195 



inconspicuous as to play no part in the characteristic 

 colours of these bodies. Such characteristic colours are, as 

 will be shown, due to the same cause as the colours of thin 

 plates in all these structures almost without exception. The 

 colours of these bodies, all, therefore, obey the laws which 

 regulate the change of tint in thin plates with varying obli- 

 quity of illumination. Thus, as the angle of incidence of the 

 illuminating light increases, or the direction becomes more 

 oblique, all such iridescent objects as feathers, butterflies, 

 beetles, flies, opal, mother of pearl, &c., &c., change in 

 colour from red towards violet in the order of the colours 

 of the spectrum. Thus, if any of this extensive group of 

 iridescent bodies, whether bird, insect, or mineral appears 

 red when the light by which it is illuminated falls on it 

 and is reflected from it at a certain angle, such body will 

 appear yellow when the angles of incidence and reflection 

 become greater, that is to say when the light is made to fall 

 on the object at a greater obliquity, and if these angles are 

 still further increased the body will appear green. Examples 

 of this are seen in the case of the crimson body of the com- 

 mon British fly, the Ruby-tail, Chrysis ignita, and many 

 other members of the same genus, the curious little beetle 

 Poropleura bacca, feathers from the crest of the humming-bird 

 Chrysolampis mosquitus or Ruby-crest, and innumerable 

 other natural objects. Again, if any of such class of bodies 

 appears yellow when the light falls on it at a small incident 

 angle, it will change to green, and then perhaps to blue as 

 the incidence becomes successively greater. I say ' perhaps ' 

 because it is not always possible, though commonly it is so, 

 to observe three changes in the same object. Examples of 

 yellow objects changing to green are met with in the 

 cases of all iridescent feathers, the colour of which is yellow 

 at normal incidence. Thus the orange throat, or gorget as it 

 is termed, of the same humming-bird Chrysolampis is seen 

 to change to green on increasing the obliquity of the 



