ACHROMATISM. 179 



great, so that the violet rays meet before the red, as 

 in a simple concave lens, the lens is said to be over- 

 corrected, and the aberration is called negative. Al- 

 though the positive chromatic aberration of the ex- 

 treme rays passing through a convex lens may be 

 corrected by the negative aberration of a concave 

 lens, there still remains a certain amount of uncor- 

 rected colour, arising from the irrationality of the 

 spectra of the two refracting media. This evil cannot 

 be overcome, and the remaining colour is said to 

 arise from the secondary spectrum. 



The object-glasses of the microscope, consisting of 

 the compound lenses, have their aberrations balanced 

 to a considerable extent on the above principles the 

 lowest combination being under-corrected, while the 

 upper combinations are over-corrected ; and, by suit- 

 able adaptation of their distance from each other, 

 further correction may be obtained, the aberration 

 of the object-glass altogether being, however, over- 

 corrected or negative. 



The eye-piece consists of two simple plano-convex 

 lenses, the upper or eye-glass (fig. 27 e) having a 

 shorter focus than the lower (/) or field-glass, and 

 the two placed at the distance of half the sum of 

 their focal lengths. The object-glass alone would 

 form an enlarged and reversed image of the object 

 within the body of the microscope, the cones of rays 

 from each point of the object terminating at the 

 larger arrows in the figure (fig. 27). But the rays 

 meeting the field-glass are brought by it to a focus 

 at the position of the smaller arrows, where they 

 form a reduced image ; and, subsequently passing 

 through the eye-glass, they are so altered in direc- 

 tion as to enter the eye at a greater angle, and to 

 present a magnified image of the object. 



The eye-piece produces several important effects. 

 The refraction being produced by two less convex 

 lenses instead of one of greater convexity, the 



