MICROSCOPIC ILLUSTRATIONS. 267 



the marginal rays are too short the chief condensation of 

 light will take place within the focus F, figure 3, at the 

 points f f f: if too long, then without the focus, at the 

 points f f f, figure 4, beyond the principal focus F. With 



these defects may be associated inequalities produced by 

 irregular working, frequently giving rise to two or more 

 curves in their surfaces. 



4thly. Object-glasses are subject to imperfections, arising 

 from an imperfect correction of their chromatic aberration ; 

 for example, they may be under or over corrected ; that 

 is, the concaves of flint glass applied to them may be 

 either of too shallow or too deep curvature to produce 

 correction, both of which defects are nearly equally inju- 



T2 



