FOREIGN OBJECTIVES. 



55 



letters, but gives also the equivalent focal lengths in inches 

 and millimetres ; they are as follow : 



In the foregoing remarks it will be observed that 

 moderate angles have been advocated for students' use ; 

 but it must by no means be inferred that moderate angles 

 are the best. The higher the numerical aperture of an 

 objective, the nearer does the front lens approach the 

 object when in focus, and this has been urged as an 

 objection against the use of large apertures. There is 

 some force in this objection in dissecting, for instance, 

 it is absolutely necessary to have room underneath the 

 objective for the use of the various knives, needles, scissors, 

 and other instruments, which one cannot get with large 

 apertures ; but when the object has been prepared there 

 is no valid reason why the best lenses should not be used 

 for its examination. 



