28 



medium powers, T 4 ^ inch, \ inch, J- inch, ^ inch ; 

 high powers, \ inch, \ inch, -^ inch, r 1 ^ inch, ^ inch, 

 2 1 inch, ^g- inch. 



As the objective is the most important of the two 

 optical parts, it follows that this must be as free from 

 faults as possible and all that human inge- 

 nuity and skill can devise is utilized to attain this 

 end. The advance in the perfection of the objective 

 has been step by step and each era was at the time 

 considered by many authorities the limit to further 

 improvement. Each advance was signalized by a 

 marked opposition and disbelief of its possibility. It 

 is therefore of inestimable credit- to the pioneer 

 objective-makers, and notably among these two Ameri- 

 cans, who by quiet but stubborn application disproved 

 previous claims and opened the way to further improve- 

 ments. A theoretical limit has been fixed on the 

 capacity of the microscope, which according to our 

 present knowledge can not even be reached. 



While the introduction of water immersion made it 

 possible to obtain higher optical results than with the 

 dry objectives, as will be explained later on, the homo- 

 geneous immersion offers still greater possibilities in 

 this direction, and the advantages are so pronounced 

 that the former are gradually coming into disuse, 

 although for certain kinds of work they will be pre- 

 ferred and used by many persons. At present homo- 

 geneous fluid is made of either thickened glycerine 

 or cedar oil, and great care is required in keeping 



