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which an eyepiece magnifies the real image. Thus 

 a 2 inch or 50.0 mm. eyepiece magnifies five times, 

 one of 1 inch or 25.4 mm. focus ten times and so on. 



Flatness of Field. Although this depends 

 mainly upon the objective, the absence of it may 

 be owing to a faulty construction of the eyepiece. 

 If it is so prominent as to be easily noticeable and 

 to the same degree with a number of objectives it 

 may be ascribed to the eyepiece. It must, how- 

 ever, be remembered that an absolutely flat field 

 has not yet been obtained ; it may be closely 

 approached by decreasing the diameter of field to 

 less than its normal size. 



Size of Field. Quite a general but erroneous 

 idea prevails that the size of the tube has an influ- 

 ence on the size of the field. Except in eyepieces 

 of very low power, or with tubes with smaller than 

 usual dimensions, this is not true. It must be 

 remembered that a Huyghenian eyepiece admits 

 of a definite size of field and this is regulated by 

 the opening in the diaphragm ; the same size of 

 opening is used in all of the same power, whether 

 it is an eyepiece with a large or small diameter. 



Defects. As has been stated many eyepieces 

 are carelessly constructed and possess defects 

 which interfere with obtaining a distinct image. 

 These defects do not show easily in low power 

 objectives, but can readily be seen with high 



