Eyepiece or Ocular. The purpose of the eye- 

 piece is to refract the diverging rays coming from the 

 objective so that they will reach the pupil of the eye 

 and at the same time magnify the image formed by the 

 objective. While it is the most simple part of the 

 optical combination of the microscope it is, withal, an 

 important part of it. In fact it may be said that, owing 

 probably to its simplicity, it has been neglected, and 

 very often eyepieces are furnished which are not at all 

 commensurate with the quality of the objectives. 



Designation. While eyepieces are still marked 

 by some makers arbitrarily without any relation to their 

 optical value, a rational system is to mark them accord- 

 ing to the equivalent focus of the two lenses. 



Thus an eyepiece marked i inch has an initial 

 magnification of ten diameters, a 2 inch, five diameters, 

 and so on. 



Field of View. As has been stated this is the 

 visible area which is limited by the size of the dia- 

 phragm. 



While the relative size of field in eyepieces of 

 different powers is the same, the actual field of view 

 as it relates to the image becomes less as the power 

 of the eyepiece increases. Thus if a i 1-2 inch will 

 show i- 10 inch of the object within the limits of the 

 field, a 3-4 inch will show only 1-20 inch. By some 



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