LIGHT-SCATTER AT POLISHED SURFACES 35 



polished — is specularly reflected along sr, and is of course brightly 

 visible to an observer who takes his view towards and along the 

 direction of sr. Some of the light is diffusely reflected in all 

 directions, d, d, d, producing, where the beam strikes, a surface 

 patch of light which is visible from whatever direction the observa- 

 tion is made, and this corresponds in size with the cross-section of 

 the beam. Thence some of the light, taking its refracted 

 direction through the substance of the glass, passes along until 



Fig. 3. — 2, thin knife-like beam passing through cornea ; 4, the 

 same through cornea near Umbus ; 3, rather wider sht-beam 

 traversing a block of glass in air. 



it meets the demarcating distal surface D where there is a 

 repetition of all three phenomena provided the glass is in contact 

 also at this surface with a medium differing in refractive index 

 from its own. Thus if a ribbon-like beam of light is passed 

 through a block of polished glass in air, a light-stripe, visible 

 from all directions, is produced by scatter at the entry and exit 

 surfaces (3, Fig. 3) where there is a change in refractive index, 

 and these stripes can be rendered much brighter by increasing 

 the relative surface-irregularity by any such means as passing a 

 greasy finger-tip over the surface. 



2 — 2 



