196 Dr. a. Hodgkinson on 



illumination, and the same is well seen in the case of 

 the outermost ring surrounding the eye of the peacock's 

 feathers. Various golden beetles and iridescent flies 

 are examples, as also various iridescent minerals, fire 

 marble liunichella, opal, &c. Examples of iridescent 

 objects, both natural and artificial, presenting at a normal 

 or small angle of incidence some shades of green, are 

 innumerable ; feathers of humming-birds, sun-birds, hosts 

 of tropical beetles and flies, and butterflies. In the 

 mineral kingdom may be mentioned opal, hunicJiella 

 labradorite, &c., and of artificially prepared bodies thin 

 films of mica and certain crystals of chlorate of potash. 

 On inclining any of these bodies, so that the illumination is 

 more and more oblique, the colour is seen to change from 

 green, through the various intermediate shades of greenish 

 blue, to blue, and then possibly to purple. Such change is 

 well seen in the outer ring of the eye of the peacock's 

 feather. Though not so common as the above, both natural 

 and artificial iridescent objects exist, which, at perpendicular 

 or normal incidence, are blue, and this, as the incident 

 angle is increased, changes to purple, and by further increase 

 in the obliquity of the illumination such objects cease tO' 

 appear coloured, reflecting white or colourless lights. Various 

 insects, more especially Lepidoptera of the genus Morphoy 

 the so-called Glory-of-Brazil butterflies, belong to this group, 

 also flies and beetles, feathers of numerous birds, mother of 

 pearl, and various mineral bodies as labradorite, specimens 

 of various ores covered with films of tarnish, thin films of 

 mica, certain iridescent crystals of chlorate of potash, &c. 

 Lastly, iridescent objects are met with which, even at normal 

 incidence, appear either violet or purple, and then, as the 

 light is made to fall on them more obliquely, simply change 

 to a higher degree of the same tint and then become white 

 or colourless as the incidence becomes still more oblique. 

 Examples of this are met with in the case of the 



