of some Brilliant Animal Colours. 101 



medium ; it commonly becomes, also, more strictly related 

 to that light which is absorbed with such great intensity." 

 But, as Walter appears to have been the first to explain, 

 there is a further important change of colour with the angle 

 of incidence, when the light-vibrations are in the plane of 

 incidence, in virtue of the abnormal refraction with its 

 accompanying abnormal polarizing angle. In the usual 

 case, where the dye is in contact with air, the polarizing 

 angle for the rays lying on the red side of the absorption- 

 band is unusually high, so that these rays, which at moderate 

 angles of incidence contribute largely to the resultant colour, 

 are extinguished at incidences of from 60° to 70°. In con- 

 sequence, the colour of the reflected light moves towards the 

 blue with increasing obliquity. 



As an example, f uchsin 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. 



" (6) 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 (7>), 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 

 Michelson * 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 



* Phil. Mag. vol. xxi. p. 554 (1911). " On Metallic Colouring in Birds 

 and Insects." 



