OBJECTIVES AND OCULARS 
13 
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 °^Eyepief ^ Negative 
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 
