244 



THEORY OF MICROSCOPIC OBSERVATION. 



2. INTERFERENCE OF REFRACTED WITH REFLECTED LIGHT. 



If p q r s (Fig. 134) is the section of a refracting body say, of 

 a crystal the parallel rays of light x and y, incident from below, 



are refracted at the surfaces q s and 

 -# P r > an( * tne latter (y) is reflected 

 also by the oblique marginal sur- 

 face. A point o in the plane of 

 adjustment M N is therefore 

 equally illuminated from two 

 ^ sources of light a and b, whose 



position is determined by the laws 

 \ of refraction and reflexion. There 

 v / is the possibility, therefore, of 

 interference, and it would be a 

 matter of no difficulty to calculate, 

 for any given case, the distance 

 of the interference lines. The 

 working out of such a calculation 

 has no special value in practice, 



since the necessary data can very seldom be determined by direct 

 observation, and must therefore be arbitrarily fixed. It is never- 

 theless evident that the interference lines seen from above are 

 projected on the surface p r, and their distances from each other 

 are the smaller the greater the distance of the source of light 

 (diaphragm) from the plane of the reflecting surface. The fine 

 linos, which in hollow cylinders take the place of the outer and 

 inner bright line as soon as the rays illuminating the margin or 

 the lumen contribute to the illumination, 1 have probably the same 

 origin as the interference phenomena in crystals here mentioned. 



1 Fkegel has made the noteworthy attempt (" Archiv fur mikr. Anat.," 

 Bd. ix. p. 507) to refer the glowing yellow-brown colour of valves of Pleuro- 

 sigma angulatum to the interference phenomena described under 1 and 2. The 

 reflexion oi the light would consequently take place at the walls of the small 

 cavities (chambers), which, according to Flcegel, cause the familiar markings. 

 The author himself considers his attempt as a mere supposition, but justly 

 insists that the phenomenon is an optical one. 



