304 



APERTURE AND OPACITIES 



CCH. IX 



FIG. 1 80. EFFECT OF APERTURE ON DEFINITE Focus AND ON OVERCOMING 



OPACITIES. 



(From Wright's Principles of Microscopy, p. 77). 



(1) To show the definiteness of the focus (/) with a large aperture. Either 

 above or below this is a large diffusion circle (a b] due to the size of the section of 

 the aperture. 



(2) Indefiniteness of the focus due to the fact that a cross section of the aperture 

 considerably above or below the true focus (/), gives so small a diffusion circle (a 

 or b) , that it can hardly be distinguished from the true focus. 



(3) Low aperture and an opacity in the path of the light. It is so large relatively 

 here that a clear image would be impossible. 



(4) The same opacity in a larger aperture. 



(5) The same opacity in a still larger aperture. There is now enough of the 

 beam outside the opacity to make the object visible. 



