1>40 THEORY OF MICROSCOPIC OBSERVATION. 



tii MI and lowering of the body-tube may be conveniently studied. 

 All real or virtual images, whose surroundings appear only faintly 

 illuminated or dark, act of course like the sources of light we have 

 mentioned for instance, the bright lines in the air-bubble and 

 the hollow cylinder, the focal lines of cylindrical threads and 

 tubes, &c. 



B. Interferences in the Plane of Adjustment. 



The interference lines observed on the edges of dark bodies, 

 air-bubbles, &c., were formerly explained as diffraction phenomena. 

 It was not considered that though the conditions of diffraction 

 obtained, which in most cases is not the fact, the resulting inter- 

 ferences are always imperceptible. For since the incident rays do 

 not proceed parallel, but form a cone of light of larger or smaller 

 aperture, the dark lines which correspond to particular inclinations 

 are always illuminated by rays of a different inclination also, and 

 the result to the eye is a uniform illumination. In all cases where 

 interference lines are perceptible, it would be useless labour to 

 exhibit the diffractive action by a construction based on measure- 

 ment ; on the contrary, we should always be assured that factors, 

 which have a modifying effect upon the diffraction phenomena, 

 exercise here either no influence at all, or an entirely different one 

 that the observed distances of the dark lines do not agree with those 

 found by construction or calculation, &c. There is, however, no 

 difficulty in referring the different phenomena in this case to 

 interferences of another kind. 



1. INTKKI KIM.M E OF DIRECT WITH REFLECTED LICHT. 



If we view under the Microscope a plane reflecting surface A 

 (Fig. 131), for instance, a thin cover-glass, or the edge of a piece of 

 metal, and allow light to fall on it through a narrow slit placed 

 somewhat laterally, there are formed at the edge of the reflecting 

 surface, and parallel with it, alternately bright and dark lines, 

 which with sufficient magnification exhibit coloured fringes. These 

 arise from interference of the direct with the reflected light. The 

 rivvin- () and its reflected image 0' are therefore to be regarded as 

 two sources of light, whose wave-systems strengthen each other at 



