100 Lord Rayleigh on the Optical Character 



behind winch the eye is placed. When daylight is used, it 

 often suffices to examine the object with one's back to the 

 window and at some distance from it. I shall have occasion 

 later to refer ajmin to Rodgkinson's work. 



The first, so far as I know, to challenge the " structure n 

 theory was Dr. B. Walter, whose tract* includes an elaborate 

 discussion, accompanied by original observations, of the 

 colours winch may arise in the act of reflexion, and decides un- 

 equivocally that the colours now in question, with one or two 

 possible exceptions, are due to surface, or quasi-metallic, re- 

 flexion as described by Haidinger, Brewster, and Stokes. The 

 first of these writers formulated a law, named after him, which 

 identifies the surface-colour with those rays which would be 

 most intensely absorbed within the substance. The theory 

 of " anomalous dispersion " since developed shows, however," 

 that the matter does not stop there, and Walter emphasises 

 that much of the surface-colour may be ascribed to rays 

 which are not themselves intensely absorbed, but being- 

 situated near an absorption-band are abnormally refracted, 

 and hence in accordance with Fresnel's laws are abnormally 

 reflected. On the red side of the band the refractive index 

 is increased and on the blue side diminished, so jhat when 

 the substance is in air the surface reflexion is redder than 

 according to Haidinger's law ; but this conclusion may 

 need to be modified when the substance is in contact with a 

 strongly refractive solid, as when a dye spread upon a glass 

 plate is examined from the glass side. In some cases it 

 appears that the surf ace-colour is due as much, or even more, 

 to these rays excessively refracted (and consequently re- 

 flected) as to those which would be intensely absorbed and 

 are reflected in accordance with Haidinger's rule. 



The departure from Haidinger's rule is specially important 

 when we consider what happens at oblique incidences and 

 with polarized light. The rays reflected in virtue of the 

 extreme opacity of the substance to them are comparatively 

 unaffected, and are indeed rendered more prominent by the 

 appropriate use of a nicol. As Stokes says f : " In the case 

 of the substances at present considered, the reflected light 

 does not vanish, but at a considerable angle of incidence the 

 pencil polarized perpendicularly to the plane of incidence 

 becomes usually of a richer colour, in consequence of the 

 removal, in great measure, of that portion of the reflected 

 light which is independent of the metallic properties of the 



* Die Oberjlachen oder Schilhrfarben, Braunschweig, 1895. 

 f Phil. Mag. vol. vi. Dec. 1858, p. 393; Math, and Phys. Papers,, 

 vol. iv. p. 42. 



