18 



CONSTRUCTION OP THE MICROSCOPE. 



the lens the focal length. The radiant point and its image 

 after refraction are called conjugate foci. These foci vary 

 according to the distance of the radiant points. In every 

 lens the right line perpendicular to the two surfaces is 

 called the axis of the lens, and is seen in the annexed 

 figure ; the point where the axis cuts the surface is called 

 the vertex of the lens. 



Fig. 5 is intended to represent the different forms of 

 lenses in use ; a is a plane glass of equal thickness 



throughout; b, a meniscus, concave on one siae, convex on 

 the other; c, a double-concave; d, a plano-concave; e, a 

 double-convex ; /, a plano-convex. 



The lenses employed in the construction of microscopes 

 are chiefly convex; concave lenses being only used to make 

 certain modifications in the course of the ravs passing 

 through those of a convex form, whereby their perform- 

 ance is rendered more exact. In accordance with the laws 

 of refraction, when a pencil of parallel rays, passing 

 through the air, impinges upon a convex surface of glass, 

 the rajs are made to converge ; for they will be bent 

 towards the centre of the circle, the radius being perpen- 

 dicular to each point of curvature. Parallel rays, falling 

 on a plano-convex lens, are brought to a focus at the dis- 

 tance of its diameter; and conversely, rays diverging from 

 that point are rendered parallel. Plano-convex lenses pos- 

 sess properties which render them valuable in the con- 

 struction of microscopes. 



Parallel rays, falling on a double-convex lens are brought 

 to a focus in the centre of its diameter ; conversely, raya 



