TEST OBJECTS. 



463 



lected and transmitted by each will be as the squares of B B 

 and b b, the focal lengths being equal. Hence it is that the 

 power of a microscope, or that faculty it possesses to render 



Fig. 258. Fig. 259. 



the structure of an object visible, depends upon the angle of 

 aperture of its object-glass, and not upon its magnifying 

 power alone. 



" But it may be supposed, perhaps, from this reasoning, that 

 if we throw a greater quantity of light upon an object, so that 

 more may be collected by the object-glass, we shall be the 

 better able to define its structure, which would probably be 

 the case if the additional light could be thrown only upon 

 those minute parts which we wish to examine, and not upon 

 the whole object. But as we cannot do this, as the increase of 

 Ulumination cannot be made to increase the relative proportions 

 of light which proceed from these minute parts, the intended 

 advantage will not be derived." 



Having now pointed out the importance of angular aperture 

 to an object-glass, when aU its aberrations are correctly 



