104 



similarly corrected. However, if they are not 

 similar in their angular aperture the one of small 

 aperture will have more penetration than the 

 other. In objectives of the same angle but differ- 

 ent power, the one of low power will have in itself 

 more penetration ; it will be similar in its action 

 to the eye, which, when an object is close to it, can 

 distinguish but one portion of it distinctly, while, 

 as its distance to the eye is increased, can dis- 

 tinguish various parts of it lying at different dis- 

 tances, and will finally see other objects outside of 

 it. By looking at an object at 5 feet distance, only 

 this can be seen plainly ; but, at 10 feet, others 

 quite a distance in front or back of it can be seen 

 as well. 



Low power objectives have a proportionately 

 greater penetrating power than medium or high 

 powers. In an object of considerable thickness, 

 different planes can be observed at one time with- 

 out focusing on them and thus obtain an apprecia- 

 tion of form which is impossible in the higher 

 powers, as in these the adjustment for different 

 depths is required. 



Furthermore the accommodation of the eye is also 

 a factor as it varies with different persons and 

 thus, to a certain extent, is a matter of individuality. 



Flatness of Field. --The field in a microscope 

 is that portion which is observed in the eyepiece, 

 and its flatness may be observed when focused on 



