OBJECTIVES AND OCULARS 



13 



Diaphragm 



magnifier of the real image formed by the objective. The image 

 seen through the ocular is formed outside the lens combinations 

 and therefore below the ocular 

 instead of between the lenses 

 and inside as we have seen is 

 the case in negative oculars. 

 Since the light rays do not 

 cross in positive oculars chro- 

 matic aberration can be ade- 

 quately corrected for only 

 through the use of combina- 

 tions of glasses of different 

 refractive index. Positive 

 oculars as a rule also yield 

 smaller fields and less bright 

 images. On the other hand 

 positive oculars because of 

 their acting as simple mag- 

 nifiers are well suited for the 

 magnification of scales, etc., 

 and are therefore employed in 

 filar micrometers, comparison 

 eyepieces, etc. 



The two lenses in the 

 simple positive ocular are of 

 equal focal length and are 

 usually so mounted that their distance apart is less than their 

 focal length. 



It is evident that the position and diameter of the diaphragm 

 in the eyepiece greatly influence the character and size of the 

 field lens image, and are thus largely responsible for the area 

 of the field of the microscope, and consequently are very closely 

 associated with the resolving power of the optical combination 

 employed. The light rays leaving the eye lens are concentrated 

 within a tiny circle, known as the eye-point, eye-circle, Ramsden 

 disk, or Ramsden circle. The designation " eye-point " has been 

 given to this smallest bright spot of light, since it is the proper 



Fig. 2. 



Path of Light Rays in a Negative 

 Eyepiece. 



