304 



APERTURE AND OPACITIES' 



CCh. IX 



Fig. i8o. Effect of Apeetuke on Definite Focus and on Oveecomino 



OPAaiiES. 



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



(i) 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 (0 

 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. 



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

 beam outside the opacity to make the object visible. 



