156 Mr. Alex. Hodgkinson on 



remain unaltered by change of position of the object. 

 This piece of Haliotis shell exhibits indescribable changes 

 of colour with every movement, but the difficulty of descrip- 

 tion, though by no means removed, is immeasurably lessened 

 by the use of the mirror. And the same with this specimen 

 of iridescent iron ore, its colours, which vary to the unaided 

 eye, remain unchanged when examined by the mirror. To 

 simplify the description of iridescent objects, therefore, 

 I would advocate the above method, and would describe the 

 result of such examination by recording the colour observed 

 by aid of the mirror, and prefixing the term "iridescent" to 

 express the changing properties of the colour. Bearing 

 in mind the unvarying nature of these changes, a 

 far clearer idea may be formed of the appearance 

 of these objects than from any attempted description of 

 what is admittedly indescribable. Time and space are also 

 economised by the omission of lengthy descriptions. The 

 accuracy, and, therefore, the value of any description of 

 colour, is always enhanced by mapping its spectrum ; more 

 especially is this true in the case of iridescent colours. 

 This is easily done, and by applying such map to a 

 spectral chart, the order of the colour, and therefore its 

 tint, is apparent. In examining many objects, chiefly birds or 

 insects, by means of the mirror as above described, apparent 

 exceptions are repeatedly met with to the fact stated above 

 that the colour is invariable in tint and disappears by 

 inclination of the body. Such instances are no real excep- 

 tions, but are due to the reflecting plates being curved, or 

 having pigmentary matter beneath them, or an opalescent 

 medium above them. In this way some of the most extra- 

 ordinary and beautiful colour effects it seems possible to 

 conceive are produced. Some of them I hope to bring 

 before your notice on a future occasion. 



In examining objects with the perforated mirror a single 

 light is necessary. The sun is of course the best, and the 



