38 



Penetration. This is the quality which enables 

 us to look into an object to observe different planes 

 at one time. In the mind of the writer, it is of no 

 special importance, or at any rate not as much as is 

 claimed for it, and if desired is easily attained. It 

 depends upon magnifying power and angular aper- 

 ture, and decreases with the increase of either of 

 these. Objectives are generally not constructed with 

 any reference to it ; it is a natural consequence of 

 certain conditions. 



Penetration and resolving power are antagonistic, 

 or at any rate in an inverse ratio, and can only be 

 combined to a certain extent. In two objectives 

 of the same power and aperture, one cannot have 

 penetration as a special feature and the other resolv- 

 ing power ; they will be almost similar in these quali- 

 ties, provided that they are similarly corrected. How- 

 ever, 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 

 different 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 distinguish 

 various parts of it lying at different distances, and 

 will finally see other objects outside of it. By look- 

 ing 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. 



