RELATIVE POWER OF OCULARS. 



43 



table will show. The numbers have been calculated from 

 the catalogues of the different makers : 



Our American brethren treat their oculars in a more 

 rational manner, they style them as "2-inch," " i-inch," 

 or otherwise, as the case may be, according to the degree 

 of amplification they yield when compared with single 

 lenses ; thus a " 2-inch ocular " would amplify the same as 

 a single lens of 2-inch focus, and so on in like proportion. 



Now reference to Chapter I. will show us that a single 

 lens of 2-inch focus (equal to the English A eye-piece) 

 magnifies about five diameters at a distance of I2j inches 

 from the micrometer to the screen, and the i-inch, 10 

 diameters (equal to the C eye-piece), so that it is well 

 the student should as early as possible grasp the fact 

 that the amplification of an object is arrived at by two 

 stages, the objective producing an enlarged image of the 

 object, which the ocular magnifies still further. Roughly it 

 may be stated that if the inch objective be used with 

 the i-inch ocular an amplification of 100 diameters 

 is arrived at, the objective magnifying the object 10 

 diameters, the image of which is further magnified 10 

 diameters by the ocular ; and further, if the same objective 



