with low power objectives ; thinner covers use with 

 oil immersion objectives, as increased working distance 

 is obtained. 



Penetration. Penetrating power is the quality 

 which enables us to look into an object to observe 

 different planes at one time. It depends upon magni- 

 fying power and angular aperture, and decreases with 

 the increase of either of these properties. Objectives 

 are generally not constructed with any reference to it ; 

 it is, rather, a natural consequence of certain qualities 

 of the objectives. 



Penetration and resolving power are antagonistic, or 

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

 bined in a definite proportion. In two objectives of 

 the same power and aperture, one cannot have penetra- 

 tion as a special feature and the other resolving power ; 

 they will be similar in these qualities provided they are 

 similarly corrected. However, if they are not similar 

 in their angular aperture, the one of smaller aperture 

 will have more penetration than the other. In objec- 

 tives of the same angle but different power, the one 

 of low power will have in itself more penetration. 



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 without focusing on them 

 and we thus obtain an appreciation of form which is 



94 



