INTERFERENCE COLOURS 



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to proceed in layers, as if it were filling inter-lamellar spaces. The 

 dimensions of the colour-producing structure are consistent with the 

 foregoing data, the lamellae of which would only be about 0-3 to 0-5 \x 

 in maximum thickness : there is space for a number of such films in 

 any of the specimens examined. 



Iridescent scales as studied by Mason are grouped by that 

 observer into three main types, each of which owes its colour to 

 multiple films separated by air, viz. : — 



1. The Urania type, in which the colour-producing lamellae are 



Fig. 64. Scale of Morpho type (schematic). A, longitudinal vanes, 

 consisting of inclined multiple films, the seat of the colour. 

 B, basal layer. (After Mason.) 



parallel to the plane of the scale and may be overlaid by a ribbed 

 or meshed structure (Fig. 63). The multiple films may reside in 

 either the upper or the lower lamina of the scale ; their plane is 

 that of the scale itself, and their optical properties are consistent 

 with that of a system of multiple thin films. The superposition of 

 a ribbed (vaned) or meshed structure on the outer surface of the 

 upper lamella of the scale modifies the visibility of the film colours, 

 but does not prevent the recognition of their typical properties. 

 This type was recognised by Onslow, prior to Mason, and these 

 two authorities are in agreement as to its chief features. 



