176 THE FUNDAMENTAL ASPECT OF COLORATION 



by detailed comparisons. Also, the histological evidence afforded 

 by scale-structure is consistent with the optical phenomena 

 exhibited. 



Mason points out that the colours of the scales are readily recognised 

 by comparison with artificial thin films, as corresponding to those of 

 Newton's series, in the second or third orders. This relationship is con- 

 firmed by the change in hue which takes place with increasing angle of 

 incidence. In every case this is towards a colour lower in order (change 

 towards violet, the colour lowest in the second order), and duplicates 



Fig. 63. Scale of Urania type (schematic). A, thin vanes or ridges 

 comprising longitudinal striations. B, multiple films, the seat 

 of the colour. C, lower lamella. (After Mason.) 



that of an artificial thin film held parallel to the reflecting surface of the 

 scale. The colour produced by swelling a scale is predictable in all cases 

 as that corresponding to an increase in order (thicker film), while by 

 compressing a scale the colour changes are predictable in all cases as 

 corresponding to a decrease in order (thinner film). The destruction 

 of colour where extreme pressure is applied to the scale corresponds to 

 bringing the lamellae into optical contact. The loss of iridescence and 

 metallic lustre, when a scale is permeated by a liquid of its own refractive 

 index, corresponds to that of thin films under similar conditions. The 

 fact that liquids markedly higher or lower in refractive index do not 

 cause total loss of colour is also consistent. The existence of lamellae, 

 more or less parallel to the plane of the scale, is borne out at the fractured 

 edges of scales : distorted sections, furthermore, may show the lamellae 

 separated. Penetration by a viscous liquid is not immediate, but tends 



