232 



THEORY OF MICROSCOPIC OBSERVATION. 



that only the two nearest will fall within the aperture of the 

 objective. 



We now come to our proper task, viz., to establish the effect 

 which these diffraction phenomena produce in the plane of the 

 real image. This may be most simply done, if we consider the 

 aperture-images in the upper focal plane, the direct one as well 

 as those due to interference, as so many (secondary) sources of 

 light, whose rays interfere, as in Fresnel's experiment with the 

 mirror. For, since these sources of light are point for point the 

 optical images of the same primary source of light, there is no 

 difference of phase between them. 



In Fig. 128, let A B be the optic axis, a the direct image of 



FIG. 128. 



the diaphragm, and a' the nearest diffraction image. From cor- 

 responding points in these two sources of light (e.g. from the 

 centre) let an arc of a circle be drawn through B (where we 

 suppose the plan of the real image), and a second one parallel to 

 the former at a distance of a wave-length ; then the point of 

 intersection P will be the point where the first bright diffraction 

 line will be found ; f or a P is evidently an entire wave-length 

 greater than a' P, and therefore the interference reaches the 

 maximum of brightness. In order to determine the distance P B, 

 we may regard the two intersecting arcs as straight lines, of which 

 one is at right angles to a B, and the other to a f B. Consequently, 

 the small triangle, whose vertex is in P, and whose base is equal 



