Nomenclature, or Rating of Objectives. 

 While for many years objectives were marked arbi- 

 trarily by makers and differently by each maker to 

 designate the power, it is now customary to nnark 

 objectives so that the figures shall indicate the true 

 optical value on the continent of Europe in milli- 

 meters and in England and this country in inches. 

 Objectives are rated according to their equivalent 

 focus. The equivalent focus of a series or combination 

 of lenses is the same as the focus of a single lens 

 having the same magnifying power as the series or 

 combination. This is also true of eyepieces. So, if 

 two combinations equal in magnifying power a single 

 lens of i inch focus they are marked i inch or 

 25.4 mm., or if a collection of four lenses is equal to a 

 lens of 1-12 inch focus it is marked 1-12 inch, or 2 mm. 

 Magnifying power increases in proportion to the 

 decrease in focal length, so that a 1-12 inch objective, 

 for instance, will give a real image twelve times larger 

 than a i inch or, with a tube length of ten inches, a real 

 magnification of 120 diameters. 



Powers. According to their magnifying power, 

 objectives are called low, medium, or high power and 

 are classified by Carpenter as follows : 



Low powers, 3 inch, 2 inch, 11-2 inch, i inch, 3-4 

 inch, 2-3 inch. 



Medium powers, 1-2 inch, 4-10 inch, 1-4 inch, 

 1-5 inch. 



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