238 THEORY OF MICROSCOPIC OBSERVATION. 



positions, I' lying to the right and a to the left) exhibits a bright 

 circle. 



Together with these phenomena, which are referable to the re- 

 union of homofocal cones of light, interference lines are also seen, 

 which are clearly due to rays that have been deflected at the 

 margin of the object. When these rays emerge from the plane of 

 adjustment, their points of intersection of course coincide with 

 the dioptric images of the marginal parts. If, on the other hand, 

 the object is brought nearer to the focal plane, the common point 

 of convergence is moved beyond the level of the objective-image, 

 and we see at this level merely the cross-section of the image-form- 

 ing cone of light, taken at a greater or less distance from the apex. 

 In consequence of this, the rays are here disposed according to their 

 inclination to the axis ; those more inclined, which pass by the edge 

 of the objective, necessarily also pass through the peripheral part of 

 the sectional surface. Of the deflected pencils of light all those 

 annul each other in the focal plane of the objective just as in 

 the production of the structure-image whose difference of phase 

 amounts to half a wave-length, or to an odd multiple thereof. The 

 sectional surface of the cone of light consequently appears to be 

 composed of alternately bright and dark circles, which, with white 

 light, show more or less distinct colours. The more intense the 

 deflected pencils, the more clearly of course will these concentric 

 interference lines appear; they are most sharply defined when 

 direct sunlight is employed. 



If we focus the Microscope to the plane in which the virtual 

 reflected image formed by the margin of the sphere appears as a 

 small central circle neglecting the indistinctness and the lesser 

 brightness it will act precisely like the virtual image of an air- 

 bubble. Window-frames, reflected in the mirror, will therefore 

 appear as straight lines ; a slight elevation of the tube causes a 

 red colouring of the centre and a bluish margin, whilst lowering- 

 it causes a bluish centre with a red margin. These colour- 



O 



phenomena are worthy of notice, since they also occur with the 

 smallest spheres of 2 3 mic. in diameter, and obviously aug- 

 ment the difficulty of distinguishing them from small cavities; 

 both indeed appear reddish if focused slightly from the medium 

 focus ; with a somewhat lower focus they appear bright, and dark 

 with a higher focus. Only where the minute spheres form a 

 distinct real image, which therefore in every case lies above the red 



