38 MICROSCOPIC ILLUSTRATIONS. 



to pass through several glasses, each of which impedes a 

 portion of the rays, whilst others are lost by reflection at 

 the surfaces, that more should remain for the purpose 

 required than when a single glass is employed. 



Every one who has considered the subject at all, will 

 understand that by a series of glasses of different media, 

 aberrations, &c. may be corrected and almost entirely 

 dismissed ; but it is not so readily perceived how a 

 compound microscope, having a series of glasses, can, 

 with less illumination, give a brighter and more vivid 

 picture, with more of the detail or minute structure of 

 an object, than can be obtained with greater illumination, 

 by a single lens. In this particular, however, consists 

 the main advantage of an achromatic. 



Let me premise that, in order to render any object 

 visible, it is necessary that rays of light should proceed 

 from it, either by reflection from its surface, or by trans- 

 mission through it, to the eye. Again, if the number of 

 rays be insufficient, the object cannot be seen, notwith- 

 standing we employ a microscope for the purpose. 

 Bearing this in mind, I will endeavour to explain how 

 an increase of angular aperture in an object-glass, 

 independent of any increase of its magnifying power, 

 will admit a greater quantity of light from any given 

 point on the surface of an object to pass through the lens, 

 so as to render the structure of the object visible. 



Let a, and «, represent two objects, in all respects 

 alike, and let us employ two microscopes, of equal 

 magnifying powers, for the purpose of viewing them. 



