Ch. IX] APERTURE AND DEPTH OF FOCUS 303 



§ 475. Depth of focus and aperture. — It is known to all workers 

 with the microscope that with objectives of low aperture it is possible 

 to change the focus rather markedly up or down without seeming to 

 lose in sharpness, while with objectives of great aperture a sharp 

 focus is almost immediately lost in focusing up or down beyond a 

 point. The reason for this is made strikingly evident by fig. 180 

 (1, 2). Let / be the most perfect focus, if one turns to a or b the 

 appearance is almost unchanged in the low apertured objective (2), 

 but the diffusion circle is very marked in the high apertured objec- 

 tive (1). Furthermore, the brilliancy of the image must be markedly 

 greater with the larger aperture (Wright, p. 77). 



§ 476. Aperture and the effect of opacities. — Between the retina 

 and the object there are many possibilities of opacities in the image- 

 producing beam of light. For example, the eye lashes, particles of 

 dirt in the tears over the cornea, besides particles on the glass sur- 

 faces. Figure 180 (3, 4, 5) show graphically the relative obscuration 

 which must result with the same opacity in beams of different aper- 

 ture. In (3) the shadow is so great that almost the entire aperture is 

 obscured, and vision made difficult or impossible. In (4) with a 

 larger aperture the shadow is not so overwhelming, and in (5) with 

 the large aperture there is still possibility of fairly good vision in 

 spite of the shadow. 



It is believed that the inevitable narrowing of the beam in high 

 power magnification and the presence of opacities in the eye form 

 the bar to resolution, and that if the apparatus and the eye could, on 

 the one hand, be free from opacities to throw shadows and thus 

 obscure the image, or on the other hand the terminal beam could be 

 opened up to make the aperture greater, the eye could discriminate 

 beyond the limits heretofore ascribed to it (Wright, Ch. XVI). 



As the higher the power of the ocular the smaller is the eye-point 

 (fig. 23-24), it is evident that any obscurities have a greater effect 

 with the high ocular. The rule to use as low an ocular as possible is a 

 good one to follow from every standpoint (Wright, p. 227). 



