462 MANIPULATION, 



ject, copied from the Microscopic Illustrations of Mr. Pritchard, 

 will be found to convey an excellent idea of its nature and 

 value. " Let me premise," says he, " 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 

 transmission through it, to the eye. Again, if the mmiber of 

 rays be insufficient, the object cannot be seen, notwithstanding 

 we employ a microscope for the purpose. Bearing this in 

 mind, I will endeavour to explain how an increase in angular 

 aperture in an object-glass, independent of any decrease of its 

 magnifying power, wiU 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 a represent two objects, in all respects alike, 

 and let us employ two microscopes, of equal magnifying 

 powers, for the purpose of viewing them. Suppose that we 

 are going to look at some spot on the surface of A or a, 

 which we will imagine to be a delicate tissue. By a well- 

 known law of light, the rays proceed in right lines, in all 

 directions from this spot, in the manner shown by the lines in 

 figs. 258-9. Suppose B B and 5 5 to be two object-glasses, 

 of equal focal lengths ; the former a single lens, of the best 

 construction, such as was used in the old compound micro- 

 scope, and the latter a lens of the newest form, termed an 

 achromatic. Now, these object-glasses will form their respec- 

 tive images at I and i, and they will be of equal dimensions. 

 But if the number of rays proceeding from A, and faUing 

 upon the single lens, B B, is not enough, when collected at I, 

 sufficiently to stimulate the eye, any minute pore, stria, 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 faU 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 BAB and 6 a 6 are the angles of aperture of 

 the respective object-glasses ; and the quantity of light col- 



