98 HOW TO SEE WITH THE MICROSCOPE. 



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 penetrating power 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 capac- 

 ity 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 

 comparatively 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 combi- 

 nation of. speed and power with endurance as cannot 

 coexist with the greatest perfection in either of the first 

 two. 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. . . . 



