Diffuse Reflexion of Light on Matt Surfaces, 215 



of possibilities which can explain the deviations, and which 

 may be mentioned here : — 



1. The law takes only rays into consideration which come 

 from the inside of the plate. Undoubtedly the diffuse re- 

 flexion of the surface is so great that it cannot be ignored. 



2. The refraction of the incident and reflected rays has 

 been disregarded. There can be no doubt that in reality the 

 rays must be refracted as they pass from one medium into 

 another and different one. 



3. von Lommel assumed that the same quantity of light 

 l.dic.dp is diffused in all directions. As regards the front half 

 of the particles (looking on to the plate) the truth of this is 

 fully established by my experiments for azimuths 0° and 

 180°. But for the hinder half there can be no doubt that 

 the quantity of light diffused by it must be less than for the 

 front half. 



Messerschmitt finds fault with v. Lommers theory because 

 the influence of polarization is left out of account. To 

 support this assertion he mentions besides his own observa- 

 tions those of Violle* respecting polarization through emis- 

 sion, As the circumstances are so widely different (molten 

 silver being operated upon) this cannot be used as an argu- 

 ment. At the outset I showed that the polarizing of the 

 light is not a quality of diffuse reflecting substances ; when 

 it occurs it is invariably the consequence of normal reflexion, 

 which is a phenomenon quite apart from diffusion. Hence 

 von Lommel was quite right in leaving polarization out of 

 the question. 



Resume of Results. 



The investigations upon compressed powders of such thick- 

 ness that no perceptible light penetrated right through them 

 lead to the following results : — 



1. Common light is not polarized by diffuse reflexion, 

 (LommeFs hypothesis.) 



2. The intensity of the light diffusely reflected under the 

 angles -feand — e is the same, or it is independent of the 

 azimuth. There is no normal reflexion. (Lommel's hypo- 

 thesis.) 



3. The law of emission by constant incidence is inde- 

 pendent of colour, or the coefficient of diffusion is independent 

 of the wave-length in the case of particles of the given size. 

 (Lommel's hypothesis). 



4. A law for the intensity of reflected scattered light 

 cannot be symmetric in reference to i and e. 



* J. Violle, Compt. Rend. cv. II. p. Ill (1887). 



