190 MANAGEMENT OF THE MICROSCOPE. 



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 resolving power, — stand in such opposite re- 

 lations 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 respec- 

 tively 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 appreciaition of the practical applications of the in- 

 strument, to estimate the merits of an object-glass by its capa- 

 bility of showing certain lined or dotted "tests," without any 

 reference to its penetrating or defining power ; as it would be if 

 a man should estimate the merits of a horse merely by the num- 

 ber of seconds within which he could run a mile, or by the num- 

 ber of pounds he could draw; without any reference, in the first 

 case, either to the weight he could carry, or to the length of time, 

 during which he could maintain his speed ; and in the second 

 case, either to the rate of his draught, or to his power of con- 

 tinuing the exertion. The greatest capacity for speed alone, the 

 power of sustaining it not being required, and burden being 

 reduced almost to nothing, is that which is sought in the Racer; 

 the greatest power of steady draught, the rate of movement being 

 of comparative little importance, is that which is most valued in 

 the cart-horse ; but for the ordinary carriage-horse or roadster, 

 the highest merit lies in such a comhination of speed and power 

 with endurance, as cannot coexist with the greatest perfection of 

 either the two first. The Author feels it the more important 

 that he should express himself clearly and strongly on this sub- 

 ject, as there is a great tendency at present, both among amateur 

 Microscopists and among Opticians, to look at the attainment of 

 that " resolving power" which is given by angular aperture, as 

 the one thing needful ; those other attributes which are of far 

 more importance in almost every kind of scientific investigation, 

 being comparatively little thought of; and he therefore ventures 

 here to repeat the remarks he made upon this subject in his re- 

 cent Presidential Address to the Microscopical Society, of the 



