46 



magnifying powers ; thus, an eye-piece marked 1 inch 

 or by a letter signifying the same, shows that it mag- 

 nifies about 10 diameters ; one of | inch, 20 diame- 

 ters, 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 eye-piece. 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 eye-piece. It must, however, be remem- 

 bered that an absolutely flat field has not yet been 

 obtained ; it may be closely approached by decreas- 

 ing the diameter of field to less than its normal size. 



Size of Field. Quite a general but errorneous 

 idea prevails that the size of the tube has an influence 

 on the size of the field. Except in eye-pieces of very 

 low power, or with tubes with smaller than usual 

 dimensions, this is not so. It must be remembered 

 that a Huyghenian eye-piece 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 eye-piece for a large 

 or small diameter. 



A misconception also exists as to the definition of 

 field. Such inquiries are often made as : " As we 

 understand it, a wide-angle objective gives a larger 

 field ? " but it does nothing of the kind. The angular 

 aperture has no bearing whatever on the size of 

 the field, The field of view, or that which is shown 

 of the object's surface, is determined by the power of 

 the objective and eye-piece. 



