ON INTERFERENCE IN THE MICROSCOPE. 



iS> 



in Fresnel's experiment with two mirrors. For, since these hght- 

 sources are point for point the optic images of the same primary 

 light-source," there can be no difference of phase between them. 



Fig. ^. 



Let AB be the optic axis, a the direct aperture image of the 

 diaphragm opening, and a' the the next situate diffraction image. 

 If from corresponding points of these two secondary light sources, 

 {e.g., from their middle points) an arc be drawn through B, which 

 we will suppose to indicate the plane of the real image, and another 

 arc parallel with the first at a distance from it equal to the wave 

 length ; then the point P of decussation will mark the spot where 

 the first bright diffraction line is effected 3 for aP is obviously 

 greater than a'P by one wave length, and consequently the inter- 

 ference effect is here a maximum of brightness. To determine the 

 distance PB we may consider the two decussating arcs as straight 

 lines, of which one stands perpendicular on aB, the other on a'B. 

 The small triangle whose point lies at P, and whose base is equal 

 to a wave length ( = X) is consequently similar to the triangle aa'B, 



and we have BP : \ : : aB : aa', whence BP=^ — V . 



aa 



Now the distance «^ is the posterior conjugate focal distance />* 



less the focal length of the objective,— that is, jO*— /. And, again, the 



