174 COMPARATIVE VALUE OF OBJECT-GLASSES. 



depends -upon it. Many Objectives are so constructed that, even 

 with a perfectly flat object, the foci of the central and of the peri- 

 pheral parts of the field are so different, that when the adjustment 

 is made for one, the other is entirely indistinct. Hence, when the 

 central portion is being looked at, no more information is gained 

 respecting the peripheral than if it had been altogether stopped 

 out. With a really good Object-glass, not only should the image 

 be distinct even to the margin of the field, but the marginal por- 

 tion should be as free from Chromatic fringes as the central 

 portion. In many Microscopes of inferior construction, the imper- 

 fection of the Objectives in this respect is masked by the contraction 

 of the aperture of the diaphragm in the Eye-piece (§ 21), which 

 limits the dimensions of the field ; and the performance of one 

 Objective within this limit may scarcely be distinguishable from 

 that of another, although, if the two were compared under an Eye- 

 piece of larger aperture, their difference of excellence would be 

 at once made apparent by the perfect correctness of one to the 

 margin of the field, and by the entire failure of the other in every 

 part save its centre. In estimating the relative merits of two 

 lenses, therefore, as regards this condition, the comparison should 

 of course be made under the same Eye-piece. 



v. It may be safely affirmed that the most perfect Object-glass 

 is that which combines all the preceding attributes in the highest 

 degree in which they are compatible one with another. But, as 

 has just been shown, two of the most important — namely, Pene- 

 trating power and Kesolving power — stand in such opposite rela- 

 tions to the Angular Aperture, that the highest degree of which 

 each is in itself capable can only be attained by some sacrifice of 

 the other ; and, therefore, of two Objectives which are respectively 

 characterized by the predominance of these opposite qualities, one 

 or the other will be preferred by the Microscopist, according to the 

 particular class of researches which he may be carrying on ; just 

 as a man who is about to purchase a horse will be guided in his 

 choice by the kind of work for which he destines the animal. 

 Hence it shows, in the Author's estimation, just as limited an 

 appreciation of the practical applications of the instrument, to 

 estimate the merits of an Object-glass by its capability of showing 

 certain lined or dotted Tests, without any reference to its pene- 

 trating or defining power, as it would be to estimate the merits of 

 a Horse merely by the number of seconds within which he could 

 run a mile, or by the number of pounds he could draw ; without 

 any reference, in the first case, either to the weight he could carry 

 or the length of time during which he could maintain his speed, 

 and in the second case, either to the rate of his draught or his 

 power of continuing the exertion. The greatest capacity for speed 

 alone, the power of sustaining it not being required, and burthen 

 being reduced almost to nothing, is that which is sought in the 

 Racer ; the greatest power of steady draught, the rate of move- 



