DIAFRAMS IN THE MICROSCOPE. 
35 
In optical systems it frequently happens that the conjugate image of an 
object serves as starting-point or object for a second set of emergent light- 
waves which are again brought to focus by a second part of the optical 
system, and a new conjugate point is established. In this last point we find 
imaged not only the details of the original object but also any other de- 
tails which the first image plane may have contributed. In the microscope 
this phenomenon of superimposed images or concatenation of conjugate 
planes is not uncommon and is of importance in determining the positions 
of the diaframs to be used. 
In Fig. 30 the path of the rays through the microscope lens system is 
sketched. Pencils of parallel rays are brought to focus by the condenser on 
the object A B; the rays emerging from points of the object are brought to 
focus by the objective alone in the image plane A'B 1 '; before reaching this 
FIG. 29. 
plane, however, the rays are intercepted by the field lens of the ocular and 
conveyed to the image A"B" '; with respect to this lens the image A'B' 
serves as object and A "B" is its conjugate image ; A "B" serves in turn as 
object for the eye lens of the ocular; its conjugate image is the virtual 
image A'"B'" which is viewed directly by the eye placed at the eye-circle 
or Ramsden disk of the ocular C'"D'" . The apertural plane of the incident 
wave-front is obviously the stop CD at the lower focal point of the con- 
denser. Its image C'D', which is located at infinity, serves in turn as object 
for the objective and is imaged in the upper focal plane of the objective at 
C"D". Owing to the construction of the objective, this is not imaged in a 
plane but approximately in a curved surface as indicated by the figure. It 
is not possible optically to correct an objective so that both the image of 
the object and the rear focal plane of the objective are plane. The image 
C"D" or exit aperture diafram of the objective is imaged in turn at C'"D'" 
by the ocular. At this point all principal rays of the system cross. The 
substage diafram CD is accordingly the entrance pupil of the microscope 
and the eye-circle (or eye-point or Ramsden disk) C'"D'" is the exit pupil. 
From Fig. 30 it is apparent that in C'" D'" or the eye-circle of the ocular 
we have imaged the substage diafram and the rear aperture diafram of the 
