1919] OF SOME BRILLIANT ANIMAL COLOURS 587
the colour of the reflected light moves towards the blue with increasing obliquity. . ..'-: -:,-
As an example, fuchsin may be referred to, a dye specially studied by Walter, who thus (p. 52) describes the surface-colour as seen from the air side:
" (a) For light polarized in the plane of incidence :
"At small angles of incidence the reflexion is yellow-green, and at increasing angles becomes ever yellower and brighter.
"(&) For light polarized perpendicularly to the plane of incidence (that is, vibrating in this plane): .
"At perpendicular incidence the reflexion is the same as under (a), and remains approximately so up to incidences of 50°. At about 60° it becomes rapidly blue-green and at 70° an almost pure blue, attaining its greatest purity at about 72°. At still greater angles the colour passes rapidly into a bright violet, and at 85° into white.
" When ordinary unpolarized light is employed, the colour of the reflexion is intermediate between (a) and (&), but always nearer to (a) than to (6) on account of the greater intensity of reflexion under (a)." .,
It is this movement of surface-reflexions towards the blue with increasing obliquity which is regarded by Walter and Michel son* as annulling the presumption in favour of the structure theory of the animal colours, which also move in this direction; and it must, of course, be admitted that the criterion is somewhat blurred thereby. Walter, indeed, maintains that thin plate colours change too much with angle to meet the requirements of the case. To this point I will return presently; but what I wish to remark at-the' moment is that with ordinary unpolarized light the surface-colours appeal1 to change too little. Neither in the case of fuchsin nor of diamond green G the second dye specially discussed by Walter,or with any other dye hitherto examinedf, have I seen an adequate change of colour without the use of- the nicol to eliminate vibrations in the plane perpendicular to that of incidence.; In the absence of a nicol there is little sign of the blue seen with it from fuchsin at 70° incidence. Much greater changes with rriore saturated colour are exhibited by the wing-eases of beetles when so examined.
As to the adequacy of the surface-colours Michelson himself remarks: "indeed, it may perhaps be objected that'the (animal) colours are far' more vivid than any of the reflexion hues of the aniline dyes, or of any other case
* Phil. Mag. Vol. xxi. p. 554 (1911). " On Metallic Colouring in Birds and Insects." t Through the kindness of Sir J. Dewar I have had the opportunity of experimenting with a good many dyes from the Badische Aniliu-Fabrik. Following Walter, I have used warm alcoholic solutions spread upon previously warmed glass plates. Latterly I have examined some more dyes, for which I am indebted to Prof. Green. In no case have I seen any considerable change of well-developed colour unless the light was polarized. 1853, p. 393; MatJi. and Pliys. Papers, Vol. iv. p. 42.tures, p. 626,1807). If the mercury be wet, boiling may be dispensed with and negative pressures of two atmospheres are easily demonstrated.