THE SIMPLE MICROSCOPE. 27 



ray carrying with, it the image of the point from 

 whence it proceeds, and as all the rays unite to form 

 an image of the object from whence they were emitted, 

 this image is brighter in proportion as there are more 

 rays united, and more distinct in proportion as the 

 order in which they have proceeded is perfectly pre- 

 served and in perfect union. The point at which 

 parallel rays meet, after passing through a lens, is 

 known as its principal focus, and its distance from the 

 middle of the lens, the focal length. The radiant 

 point and its image after refraction are known as the 

 conjugate foci. In every lens the right line perpendi- 

 cular to the two surfaces is the axis of the lens. This 

 is indicated by the line drawn through the several 

 lenses, as seen in the diagram. The point where the 

 axis cuts the surface of the lens is termed the vertex. 



Parallel rays falling on a double-convex lens are 

 brought to a focus in the centre of its diameter ; con- 

 versely, rays diverging from that point are rendered 

 parallel. Hence the focus of a double-convex lens will 

 be at just half the distance, or half the length, of the 

 focus of a plano-convex lens having the same curvature 

 on one side. The distance of the focus from the lens 

 will depend as much on the degree of curvature as 

 upon the refracting power (called the index of refrac- 

 tion) of the glass of which it may be formed. A lens 

 of crown-glass will have a longer focus than a similar 

 one of flint-glass ; since the latter has a greater refract- 

 ing power than the former. For all ordinary practical 

 purposes, we may consider the principal focus as the 



