220 



THEORY OF MICROSCOPIC OBSERVATION. 



The optical action of elevations and depressions remains ap- 

 proximately the same in the Microscope, if the undulating form of 



the bounding surface passes into acute 

 serrations (Fig. 120) ; for edges projecting 

 outwards always produce real images, and 



FIG. 120. 



those indented virtual ones. Such forms 

 seldom occur in vegetable organisms ; they are more often observed 

 in crystalline films, e.g., in those chains of (imperfect) crystals of 

 common salt, resembling the plan of a fortress. 



FIG. 121. 



3. MEMBRANES BOUNDED BY PARALLEL UNDULATED SURFACES. 



A wave-like membrane, as represented in section in Fig. 121, 

 may be divided into groove-shaped portions of hollow cylinders 



which turn their convex and con- 

 1* cave sides upwards alternately- 

 ;/ This division is exhibited in the 

 figure by straight lines, which 

 unite the centres of curvature, as 

 well as by different shading of the portions of the membrane. 

 Since the hollow cylinder acts optically as a concave lens, such 

 portions also act dispersively upon the incident light. The two 

 principal points coincide with the centre of curvature, if the 

 surrounding medium is air, but become separated, and at the same 

 time approach the refracting surfaces, if the surrounding medium is 

 water or other liquid. Tn regard to this point and the magnitude 

 of the focal lengths, the following table exhibits a few examples 

 which have been calculated. By e and e' are denoted the distances 

 of the principal points from the centres of curvature, by <j> the 

 focal length, by r the radius of the stronger curvature, and by n 

 the absolute refractive index. The radius of the lesser curvature 

 (R) is taken as standard of unity, and the surrounding medium 

 water with the index . 



