28 THE MICROSCOPE. 



focns for parallel rays is termed of a double-convex 

 lens to be at the distance of its radius, that is, in its 

 centre of curvature ; and that of a plano-convex lens 

 to be at the distance of twice its radius, that is, at the 

 other end of the diameter of its sphere of curvature. 

 The converse of all this occurs when divergent rays 

 are made to fall on a convex lens. Kays already con- 

 verging are brought together at a point nearer than the 

 principal focus ; whereas rays diverging from a point 

 within the principal focus are rendered still more 

 diverging, though in a diminished degree. Bays 

 diverging from points more distant than the principal 

 focus on either side, are brought to a focus beyond it : 

 if the point of divergence be within the circle of curva- 

 ture, the focus of convergence will be beyond it ; and 

 vice versa. The same principles apply equally to a 

 plano-convex lens ; allowance being 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 is found by multiplying 

 the radius of one surface by the radius of the other, 

 and dividing this product by half the sum of the radii. 



The refracting influence of concave lenses will be 

 precisely the opposite of that of convex. Bays 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 will be, for a piano- 

 concave lens, at the distance of the diameter of the 

 sphere of curvature ; and for a double-concave, in the 

 centre of that sphere. A lens convex on one side and 

 concave on the other, is known as a meniscus. 



In the construction of the microscope, either simple 

 or compound, the curvature of the lenses employed is 

 spherical. Convergent lenses, however, with spherical 

 curvatures, have the defect of not bringing all the 

 rays of light which pass through them to one and 

 the same focus. Each circle of rays from the axis 

 of the lens to its circumference has a different focus, 

 as shown in fig. 14. The rays a a, which pass 

 through the lens near its circumference, is seen to bo 

 more refracted, or come to a focus at a shorter distance 



