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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 



