of some Brilliant Animal Colours. Ill 



very close to a large window facing a nearly uniform sky, 

 the intervals between the spots filled up with colour, for the 

 most part of approximately the same hue, and the reflexion 

 was nearly uniform except for the small round holes already 

 mentioned. Evidently the reflexion of the gas-mantle had 

 failed to reach the eye, except from a relatively small area 

 presenting the proper angle, thus explaining the spotty 

 appearance observed with this illumination. 



The colours reflected at moderate angles seem highly 

 saturated. At perpendicular incidence the prism shows 

 next to nothing beyond the uninterrupted red and red- 

 orange, and on inclination the green region appears well 

 isolated. The impression left upon my mind is that the 

 phenomena cannot plausibly be explained as due to surface- 

 colour, which in my experience is always less saturated 

 than the transmission colour, and that, on the other hand, 

 the interference theory presents no particular difficulty, 

 unless it be that of finding sufficient room within the 

 thickness of the cuticle. But the alternations cannot be 

 those of plane strata, extending without interruption over 

 the whole area of the colour. 



As regards the difficulty of finding room sufficient for an 

 optical structure of the kind contemplated, Mr. Onslow 

 estimates the available thickness at from 0'75 fu, * to 2 //, in 

 the case of many butterfly scales, and this is little enough. 

 Even the larger estimate would amount to only about 9 or 

 10 half wave-lengths, even when allowance is made for the 

 wave-length being less than in air, and the lower limit 

 would apparently not suffice. But these measurements are 

 not easy, and may perhaps be disturbed by refraction effects. 

 Mr. Onslow has shown me many drawings of sections in 

 planes perpendicular to the surface from many butterflv 

 scales and from two or three beetle wing-cases. Most of 

 these exhibit structures approximately periodic along the 

 surface, but in no case a structure periodic in going inwards 

 along the normal. But a structure of the latter kind adequate 

 to the purpose may probably lie close upon the microscopic 

 limit, unless, indeed, it could be made evident in a section 

 cut very obliquely. 



It must be confessed that much still remains to be effected 

 towards a complete demonstration of the origin of these 

 colours. Even if we admit an interference character, 

 questions arise as to the particular manner, and there are 

 perhaps possibilities not hitherto contemplated. 



November 20, 1918. 



^-iOOD 



