103 



" This system involves four conditions : 



First- -That all cover glasses be measured before 

 using them, and that the thickness be noted on the 

 preparation. 



Second- -That for convenience all draw-tubes be 

 marked in inches or millimeters, or both. 



Third- -That adjustable objectives be corrected 

 according to this scale. 



Fourth- -That the same tube length and cover 

 glass thickness be used in all original corrections 

 of objectives." 



Penetration. Penetrating power 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 aperture, 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 antago- 

 nistic, 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 resolving power ; they will be almost 

 similar in these qualities provided that they are 



