THE MICROSCOPE. 129 



pie, one mm. is magnified so as to measure 100 mm., it is 

 evident that this magnification of the objective and eye- 

 piece is 100. 



The student should distinguish between linear and 

 superficial magnification. The former is always meant in 

 scientific work, whereas, the latter is employed for popu- 

 lar purposes. As an example, corresponding to the above, 

 a one mm. square would be magnified to a 100 mm. square. 

 The linear magnification would be 100; the superficial, or 

 area magnification would be 10,000. 



2. — A good defining power. — It is obvious that a good 

 objective should give as fiat a field as possible. Perfect 

 flatness of the field is not attainable, but a great deal can 

 be done by reducing the spherical aberration. The defini- 

 tion of an objective is likewise increased by properly cor- 

 recting the chromatic aberration. Moreover, the perfect 

 centering of the lenses is necessary to the good perform- 

 ance of an objective. The flatness of field can best be 

 tested by a stage micrometer, whereas the definition proper 

 can be tested by means of stained bacteria, such as the 

 tubercle bacillus. 



3. — A good resolving and penetrating power.— {Sj the re- 

 solving or delineating power is meant the ability of a lens 

 to show up fine markings and delicate structures.) A normal, 

 unaided eye may recognize divisions on a scale which 

 are sir of an inch apart. Another eye may fail to resolve 

 these lines, whereas a third may be able to recognize even 

 a smaller fraction of an inch. Again, on account of 

 "acuteness of vision", one person may make out, with 

 the unaided eye, a double star, which another person 

 may fail to do. The resolving power, therefore, and not 

 its magnifying power, is what imparts value to an objec- 

 tive. 



The resolving power of an objective depends primarily 

 upon the amount of light which it admits. The light that 

 enters is included in the angle made by the two extreme 



