Optical Character of some Brilliant Animal Colours. 99 



•able to show that the appearances follow from interference 

 of light, due to the presence of films of liquid between the 

 lamellae of the lower layer. The microscope shows brilliant 

 red and green tints by reflected light, while in transmitted 

 light the complementary colours are distinct, but without 

 brilliancy. The latter colours are seen to change when 

 pressure is applied to the surface of the cuticle, and when 

 the process of drying is watched under the microscope, owing 

 in both cases to the liquid films becoming thinner. In the 

 dry cuticle the solid lamellae probably come into contact, and 

 prevent the admission of air, which, if present, would cause 

 even greater brilliancy than liquid. The spectroscope shows 

 broad interference-bands in the transmitted light, which 

 change their position on altering the angle of incidence of the 

 light which passes through the cuticle. Preciselv similar 

 colours,, metallic on reflexion, non-metallic and with com- 

 plementary tints on transmission, with the same spectroscopic 

 appearances and changes induced by the same means, are 

 seen in the surface films which are formed on bottle-glass 

 after prolonged exposure to earth and moisture. In the 

 alternating layers of the pupa the chitinous lamellae are of 

 higher, t'ie liquid films of lower refractive index ; hence 

 water or alcohol produces brilliant appearances, while liquids 

 of higher refractive indices produce less effect." 



I owe to Prof. Poulton the opportunity of repeating some 

 of these observations, such as the loss of metallic appearance 

 on drying and of recovery under alcohol. On substitution of 

 benzol with a little bisulphide of carbon for alcohol, the sur- 

 face became very dark, but regained the golden glitter on 

 going back to alcohol. 



Of a specimen of another kind Prof. Poulton writes that 

 the bug has been in the Oxford Museum Collection for 30 

 or 40 years judged by the pin. It is brown when dry, but 

 when soaked in water becomes green like a leaf with bright 

 iridescent green stripes on the under side. This observation 

 also I have been able to repeat. All of which, it need hardly 

 be said, is strongly suggestive of interference. 



Dr. A. Hodgkinson also has described interesting- obser- 

 vations. In his early papers* he distinctly refers the colours 

 to Newton's scale, which in strictness would imply a limi- 

 tation to a single thin plate. He emphasises the importance, 

 tor purposes of identification, of recording the colours of 

 feathers &c. as seen by perpendicular reflexion, a condition 

 best secured by illumination from a small perforated mirror, 



* Manchester Memoirs, 1889; 1892, p. 149. 

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