OP THE MICROSCOPE. 65 



modern objective, for although great improvements have been 

 made within the past few years, it is upon the results of their 

 labors that these improvements have been based. And yet, 

 notwithstanding this well-known fact, the names of these 

 distinguished microscopists are not so much as mentioned 

 in this connection in the recent work of Dr. Frey, which has 

 been lately translated into English, and extensively circulated 

 in this country ! 



In estimating the quality of an objective, there are certain 

 features to which especial attention must be given. Aside 

 from magnifying power, which, of cjurse, cannot be regarded as 

 affecting the quality of an objective, these points are: 1. Defin- 

 ing power; 2. Achromatism; 3. Freedom from aberration 

 of form; 4. Flatness of fie'd; 5. Angular aperture; 6. Pene- 

 tration; 7. Working distance.* 



Defining Power. This is undoubtedly the most impor- 

 tant quality to be sought for in objectives. A glass that is 

 deficient in this point is absolutely worthless. Want of de- 

 fining power is shown by a general haziness and thickening of 

 the outlines, together with a want of clearness in the details. 

 It arises from the presence of either spherical or chromatic 



*The authors of the Mierographic Dictionary enumerate the following 

 points as those in which object-glasses differ from each other: 1. Magni- 

 fying power. 2. Defining power. 3. Penetrating power. 4. Their cor- 

 rective adaptations. The functions attributed to " defining po\vtr"are 

 the same as those given by other writers; "penetrating power "-seems to 

 be equivalent to what is generally called "resolving power;" "corrective 

 adaptation" is merely the presence of a means of adjusting for thicknc ss 

 of glass cover. Frey distinguishes two attributes of object-glasses, viz., 

 defining power, and penetrating or resolving power penetrating power 

 and resolving power being considered by him to be the same thing. 

 Carpenter enumerates four distinct attributes of object-glasses, viz., 

 " (1) Defining power, or the power of giving a clear and distinct image of 

 all well marked features of an object, especially of its boundaries; (2) itt 

 penetrating power or focal depth, by which the observer is enabled to look 

 Into the structure of objects; (3) its resolving poicer, by which it enables 

 closely approximated markings to be distinguished; and (4) the flatness 

 of the fietd which it gives." We cannot regard any of these classifica- 

 tions, as strictly logical. Beale makes no formal statement, but gives 

 some very excellent practical directions in regard to the selection of ob- 

 jectives. 



