22 CONSTRUCTION OF THE MICROSCOPE. 



In optical instruments the curvature of the lenses 

 employed is spherical, that being the only form which 

 can be given by grinding with the requisite degree of 

 truth. But convergent lenses, 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. 9. The rays a a, 



Fig. 9. 



which pass through the lens near its circumference, it is 

 seen to be more refracted, or come to a focus at a shorter 

 distance behind it than the rays 6 6, which pass through 

 near its centre or axis, and are less refracted. The conse- 

 quence of this defect of lenses with spherical curvatures, 

 which is called spherical aberration, is that a well defined 

 image or picture is not formed by them, for when the 

 object is focused, for the circumferential rays, the picture 

 projected to the eye is rendered indistinct by a halo or 

 confusion produced by the central rays falling in a circle 

 of dissipation, before they have come to a focus. On the 

 other hand, when placed in the focus of the central rays, 

 the picture formed by them is rendered indistinct by the 

 halo produced by the circumferential rays, which have 

 already come to a focus and crossed, now fall in a state of 

 divergence, forming a circle of dissipation. The grosser 

 effects of this spherical aberration are corrected by cutting 

 off the passage of the rays a a. through the circumferences 

 of the lens, by means of a stop diaphragm, so that the 

 central rays, b b, only are concerned in the formation of 

 the picture. This defect is reduced to a minimum, by 



