AMPLIFYING POWER. 



53 



object, under varying positions of the screw collar, such as 

 Gyrosignia angulatum or the Podura scale, until the best 

 definition has been arrived at. 



Under the denomination of high powers may be classed 

 all those yielding greater amplification than the J-inch. 

 They have become very common during the last few years, 

 nearly every maker of objectives issuing an " eighth," and an 

 immersion ^ or ^. 



It appears probable that most of the high powers now 

 made are constructed upon a formula similar to that last 

 described by Mr. Wenham, and illustrated in .Fig. 31 

 (though this is denied by some makers), as he states most 

 distinctly that the formula is successful with all powers 

 requiring adjustment for cover-glasses, " from the J-inch 

 upwards." 



Eighths and higher powers used to be specialities of the 

 first-class makers for which very high prices were charged ; 

 but now, for a student's general work, very good eighths 

 may be purchased for 50^., of an aperture of 0-82 (110) 

 and good working distance. 



As to magnifying power, the annexed list has been ex- 

 tracted from the catalogue of Messrs. Powell and Lealand, 

 and will serve to show what degree of amplification is 

 yielded by the various objectives, in combination with 

 different oculars, at the recognised distance of 10 inches. 



