40 MICROSCOPIC ILLUSTRATIONS. 



not enough, when collected at i, sufficiently to stimulate 

 the eye, any minute pore, striae, or other marking at a, 

 will not be rendered visible ; whilst, from the increase 

 of aperture in the achromatic lens, b, b, allowing much 

 more light from a, to fall upon it, and to be transmitted 

 through it and collected at i, every marking, &c. at a, 

 will be clearly represented at i ; and the eye, being 

 powerfully acted upon by this increase of light, will 

 become highly sensible of it. 



The angles b, a, b, and b, a, b, are the angles of aperture 

 of the respective object-glasses ; and the quantity of 

 light collected 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 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 illumination cannot be 

 made to increase the relative proportions of light which 

 proceed from these minute parts, the intended advantage 

 will not be derived. 



Having shewn, however, that the goodness of a 



