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marked 1 inch, or if a collection of four lenses will 

 be equal to a lens of ^ inch focus it is so marked. 

 The powers increase in proportion to the decrease 

 in focal length, so that a -fa for instance will give 

 a real image twelve times larger than 1 inch or a 

 real magnification of 120 times. 



Powers. According to their powers, objectives 

 are called loiv, medium or higJi and are classified by 

 Carpenter as follows : 



Low powers, 3 inch, 2 inch, 1^ inch, 1 inch, f 

 inch, f inch. 



Medium powers, \ inch, T 4 F inch, J- inch, \ inch. 



High powers, \ inch, \ inch, y 1 ^ inch, fa inch, 

 inch, -fa inch. 



It might be stated that such powers as ^ and 

 inch are very rarely constructed at the present 

 time and that the -fa inch may be considered the 

 maximum, although seldom used. The fa inch is 

 the highest which is ordinarily used and will give 

 all the optical advantages, while the higher powers 

 involve so many mechanical difficulties as to 

 increase the cost of production very considerably 

 and as a rule detract from the optical qualities. 

 A modern objective of the highest capacity may 

 be considered a work of art and there are few pro- 

 ductions of the human hand which exact so much 

 untiring application, ingenuity and skill. 



Systems. An objective is said to consist of 

 systems which may vary in number from one to 



