12 
ELEMENTARY CHEMICAL MICROSCOPY 
object formed by the objective; but in addition to this the usual 
type of ocular employed serves as a collector of light rays and 
increases the brilliancy of the image and therefore of the useful 
area of the field of view. 
Eyepieces are of two types, those in which the real image is 
formed inside the lens system of the ocular, and those in which 
the real image is formed outside the ocular. The former are 
known as negative or Huy genian eyepieces; the latter, as positive 
or Ramsden eyepieces. 
Oculars are designated either by their equivalent focal length, 
by the number of times they magnify the real image formed by 
the objective or by arbitrary numbers or letters based upon 
either equivalent focus or magnification. The shorter the equiv¬ 
alent focal length the higher the magnification. When desig¬ 
nated by their magnification the figures with which they are 
marked indicate the number of times the real image is magnified. 
The negative or Huygenian ocular is almost universally 
employed in microscopic work. It consists of two plano-convex 
lenses mounted convex sides down. Through this construction 
the lower or field lens becomes optically a part of the objective 
system since it collects the light rays and reduces the size of 
the real image formed by the objective. This leads to the pro¬ 
duction of a brighter image as seen in the microscope, increases 
its clearness and because of the reduction in size of the real 
image the field of the microscope is enlarged. It will be seen 
on consulting the diagram. Fig. 2, that the light rays cross just 
above the field lens, this yields, to a considerable degree, a cor¬ 
rection for chromatic aberration without the use of combinations 
of flint and crown glass. 
The lenses in the negative ocular are usually so placed in their 
mounting that their distance apart is about half the sum of their 
focal lengths. Theory calls for a focal length of the field lens 
to be about three times that of the eye lens. In practice this 
combination rarely obtains. 
The positive or Ramsden ocular consists of two plano-convex 
lenses with their convex surfaces turned toward each other (see 
ocular shown in Fig. 27) and the entire combination acts as a 
