42 



better impression of a large, coarse object and its 

 relative parts, not only because it makes a larger sur- 

 face visible, but because it has more penetration. 



In objectives of the same power, but of different 

 angular aperture, the magnifying power and field will 

 always be the same. 



The following table which has been compiled will 

 probably be of assistance to the beginner. After he 

 has become better acquainted with his instrument his 

 judgment will dictate to him what to do. 



A power of 25 diameters will show a surface of 

 about J inch diameter. 



A power of 50 diameters will show a surface of 

 about T L inch diameter. 



A power of 100 diameters will show a surface of 

 about gV inch diameter. 



A power of 500 diameters will show a surface of 

 about yi^ inch diameter. 



A power of 1000 diameters will show a surface of 

 about TjJ-Q- inch diameter. 



This table is approximately correct with a Huy- 

 ghenian eye-piece ; with a Periscopic almost double 

 the amount of such surface will be shown. 



Magnifying power may be obtained by the eye- 

 piece or objective andthe desirability of using one or 

 the other for this purpose was for many years a mat- 

 ter of spirited discussion, but it is now generally con- 

 ceded that increased power should be obtained by 

 increasing the power of objectives and not go beyond 

 the equivalent power of 1 inch in the eye-pieces. 



Objectives of the same angular aperture, but of 

 different power, will give identical results by bringing 

 them up to the same magnifying power, unless the 



