INTERFERENCE COLOURS 175 



Collodion impressions retain the same pattern, and the iridescence 

 exhibited by these impressions is as good as that shown by the 

 insect itself. All the properties exhibited are in accord with 

 those of diffraction gratings and are not manifested by thin films 

 or any other colour-producing structure known. 



Interference. The majority of recent investigators are in 

 favour of the interference theory as being the one which explains 

 by far the largest number of cases of iridescent colours. In the 

 wings of many insects colours are exhibited which recall the 

 iridescence of soap bubbles. It is obvious that these colours are 

 structural, since they prevail not only in entirely unpigmented 

 wings, but also in pigmented wings after subjection to bleaching. 

 When examined by transmitted light, an iridescent insect wing 

 exhibits only very pale hues which are complementary to those 

 shown in the same area by reflection. Immersion in liquids of 

 the correct refractive index largely destroys such colours, 

 which become restored after thorough washing and drying. The 

 iridescence is not destroyed by the separation of the upper and 

 lower wing lamellae, while collodion impressions of either surface 

 show no iridescence. Mason, who has paid some attention to 

 iridescent wing membranes, concludes that the phenomenon is 

 explainable on the basis that the colours are due to multiple thin 

 films separated by material of slightly different refractive indices. 

 He maintains that microscopical demonstration of an actual 

 laminated structure in the upper or lower cuticle is hardly to be 

 hoped for, since the thinness of the films as calculated (circa 0-2 /x) 

 must be about at the limit of resolution. Great difficulties would 

 be involved in making microscopical preparations sufficiently 

 reliable to avoid the possibility of spurious diffraction images. 

 He concludes, therefore, that optical tests are more reliable, in 

 this case, than histological study, and it may be added that the 

 tests as applied by him afford definite support to his conclusions. 



The explanation of scale iridescence is not so obvious as in the 

 case of wing-membranes, and the subject has received a good deal 

 of attention by all writers on the structural colours of insects. The 

 striking resemblances between the optical properties of insect scales 

 and of thin, colour-producing, artificial films, can be substantiated 



