by Diffusion of Light, 57 



ference ; these phenomena are most frequently only superposed 

 to those which are realized in a more general manner. 



A black body is completely opaque. It is seen only by super- 

 ficial reflection : if the surface is polished, the reflection is spe- 

 cular ; if the surface is dull, there is diffusion according to the 

 laws we have ascertained concerning polarization. Between 

 absolute polish and perfect absence of lustre we may have all the 

 intermediate degrees. 



A white body is transparent for every sort of luminous rays. 

 It is visible, first, in consequence of superficial reflection, exactly 

 as the black body ; but, in addition, there arrive at the eye rays 

 which have undergone one or more interior reflections. If the 

 body is polished and homogeneous, we have the well-known ap- 

 pearance of glass or colourless crystals. If the body is dull and 

 homogeneous (ground glass), we have phenomena of surface 

 diffusion as for black bodies, but with a twofold complication : 

 the light which has penetrated the interior emerges again either 

 directly or after interior reflection, undergoing diffusion by re- 

 fraction ; moreover the light diffused exteriorly falls in part on 

 the roughnesses of the surface, and may pass through them. If 

 the body is polished but not homogeneous (porcelain), the sur- 

 face reflection is specular; but the rays which penetrate the 

 substance are reflected or diffused in the interior, and a great 

 portion of them emerge from the body. Finally, if the body is 

 dull and not homogeneous, we have the combination of the two 

 preceding cases. 



It is easy to understand that the phenomena of polarization 

 are completely disturbed as soon as the diffusion is no longer 

 merely superficial. A very simple experiment makes this im- 

 mediately intelligible. We have seen that, when a horizontal 

 pencil polarized in a vertical plane falls on a plate of glass covered 

 with lampblack, the visual plane being horizontal, and the visual 

 angle 90 c , we have only a residue of neutral light, whatever may 

 be the position of the diffusing surface in relation to the incident 

 pencil. If we turn the glass plate so that the pencil must pass 

 through the glass in order to arrive at the lampblack, then the 

 phenomenon is completely changed : the eye remaining in the 

 same position, receives, instead of a neutral residue, light strongly 

 polarized. The reason is easy to comprehend : the pencil 

 of light, penetrating the plate (which is necessarily inclined 

 upon its direction), is refracted and deviates; the lampblack 

 therefore does not receive a pencil making a right angle 

 with the visual ray; and therefore the light diffused in the 

 direction of the latter must be polarized. To this cause of 

 perturbation is joined the change of plane of polarization 

 which the incident ray generally undergoes by its refraction, 



