94 HOW TO SEE WITH THE MICROSCOPE. 



a, perpendicular section of the lowest combination of an 

 object glass of small aperture. 



"a is the angle of aperture, and f e the most oblique 

 rays which the object-glass will admit. The angle is 

 measured by the dotted arc b in the object-glass of 



larger aperture (Fig. to the right) ; the arc 6, which 

 measures the angle, is much larger, and the radii repre- 

 senting the extreme lateral rays are much more oblique. 

 Hence it is evident that the object-glass of larger aper- 

 ture admits all those rays admitted by that of less 

 aperture, and a certain number of other rays, these 

 being more oblique." 



The above definition has been and is now generally 

 accepted. Messrs. Griffiths & Henfrey proceed to give 

 their demonstrations of the effect of aperture, as thus 

 defined, in an interesting and lengthy article, one that 

 is well worth perusal by those desiring to know how 

 things stood twenty-two years ago. Our limited space 

 forbids its reproduction here. 



