OPTICAL PRINCIPLES OF THE MICROSCOPE. 



Rays diverging from the farther extremity of 

 one diameter of curvature of a double-convex lens, 

 brought to a focus at the same distance on the 

 other side. 



equal distance on the other side of the lens (Fig. 4); but the more the 

 point of divergence is approximated to the centre or principal focus, the 

 farther removed from the other 

 side will be the point of conver- 

 gence (Fig. 5), until, the point 

 of divergence being at the cen- 

 tre, there is no convergence at 

 all, the rays being merely render- 

 ed parallel (Fig. 3); whilst if 

 the point of divergence be be- 

 yond the diameter of the sphere 

 of curvature, the point of con- 

 vergence will be within it (Fig. 

 5). The farther removed the 

 point of divergence, the more 

 nearly will the rays approach 

 tne paralJel direction i until, at 

 length, when the object is very 

 distant, its rays in effect become 

 parallel, and are brought together in the principal focus (Fig. 3). If, on 

 the other hand, the point of divergence be within the principal focus, 

 they will neither be brought to converge, nor be rendered parallel, but 

 will diverge in a diminished de- 

 gree (Fig. 6). And conversely, 

 if rays already converging fall 

 upon a double-convex lens, they 

 will be brought together at a 

 point nearer to it than its centre 

 of curvature (Fig. G). The same 

 principles apply equally to a 

 plano-convex lens; allowance be- 

 ing made for the double distance 

 of its principal focus. They also 

 apply to a lens whose surfaces 

 have different curvatures; the 

 principal focus of such a lens be- 

 ing found by multiplying the 

 radius of one surface by the rad- 

 ius of the other, and dividing this and vice versa. 

 product by half the sum of the 

 same radii. The rules by which 

 the foci of convex lenses may be 

 found, for rays of different de- 

 grees of convergence and diver- 

 gence, will be found in works on 

 Optics. 



6. The refracting influence of 

 concave lenses will evidently be 

 precisely the opposite of that of 

 convex. Rays which fall upon 

 them in a parallel direction, will 

 be made to diverge as if from the 

 principal focus, which is here 



Called the negative foCUS. This in a dimiriisheU'degree. 



diverging from points more distant than 



