High powers, 1-6 inch, 1-8 inch, 1-10 inch, 1-12 

 inch, 1-16 inch, 1-20 inch, 1-25 inch. 



t 

 It might be stated that such powers as 1-20 and 



1-25 inch are very rarely constructed at the present 

 time and that the 1-16 inch may be considered the 

 maximum, although seldom used. The 1-12 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 rather 

 detract from than add to the optical qualities. 



While the above list of objectives comprises the 

 variety of powers generally offered by makers, exper- 

 ience in the different courses of study has shown that 

 certain objectives, and these generally in sets, are 

 most suitable. Thus with the short tube the 2-3 is 

 generally used for a low power, with the 1-6 and less 

 frequently the 1-8 for a medium, and the 1-12 oil 

 immersion for a high power. With the long tube the 

 3-4 and 1-5 are usually selected with the 1-12 for a 

 high power. When the work requires low and medium 

 powers only, the 11-2 and 1-2 are preferable. Of the 

 low powers the 2, i 1-2 or i inch are used. The 3 inch 

 and lower powers are rarely required, except in photo- 

 graphy, as the magnifying power is not much greater 

 than that obtained by a good pocket magnifier. 



