56 THEORY OF THE MICROSCOPE. 



zone remains over-corrected in defective constructions, and all 

 further approximation or ' separation of the lenses will only 

 move the residuum of aberration between the centre and the 

 margin, so that either this or that zone is temporarily more or 

 less free from aberration, the one always at the expense of the 

 other. In such cases more cannot be attained even by special 

 " correction " contrivances or similar means ; the difference of 

 correction still remains, as it is based on the refractive powers 

 of the lower lenses of the objective, and cannot therefore be 

 removed by any less strongly refracting lenses placed above them. 

 The real capacity of the Microscope can never be increased by 

 such means. The advantages they furnish may be just as well 

 secured by proper construction of the objective itself, and the 

 errors which cannot be thus overcome remain notwithstanding 

 the application of correction-adjustments. 



The mode in which the optician has to combine all the factors 

 which influence the path of the rays, in order to produce as 

 favourable a result as possible, cannot be fixed once for all, 

 however much the general laws of the course of the rays be 

 studied. To come as near as possible to perfect aplanatism is, 

 as a rule, left to experimental trials by alteration of the 

 lens-distances, and by happy selection of the single lenses. 

 Opticians of the older school endeavoured to attain this end 

 to some extent by making each pair of lenses aplanatic ; modern 

 opticians, however, direct their attention to the total effect, and 

 correct the aberrations of the anterior lenses by the opposite 

 aberrations of the posterior ones. This last method leads, un- 

 doubtedly, to a higher degree of perfection, especially with high 

 powers ; the former, however, affords certain practical advantages 

 for low powers. When each pair of lenses is aplanatic, the 

 last pair may be used alone or in combination with the next, as 

 the object-point then exactly replaces the virtual image formed 

 by the front- lens, and there is consequently no derangement of 

 the aplanatism. The anterior pairs, after the removal of the 

 others, can obviously give only an indistinct image. 



It is clear, from what has been adduced, that similarly-de- 

 signated objectives as to power, even from the same manufactory, 

 can only agree approximately. In consequence of the impos- 

 sibility of reproducing the same curvatures, the optician must 

 actually test every combination of lenses, and when he has 



