34 METHODS OF PETROGRAPHIC-MICROSCOPIC RESEARCH. 
DIAFRAMS IN THE MICROSCOPE. 
In tracing the course of a wave impulse from any point in the object 
through the lens system of the microscope, the effect of the centered lens 
mounts and other diaframs, including the pupil of the eye, in limiting the 
wave-front is an important factor to be considered; these stops not only 
restrict the aperture of the effective wave-front, but also determine the 
extent of the object reproduced in the image. 
Let Fig. 28 represent a lens system which consists of two component 
lenses L\ and /. 2 , and is corrected for spherical and chromatic aberrations and 
the sine condition ; let A B be an object and A "B" the final image produced 
by the lens system; let CD be a centered stop and C'D' and C"D" its con- 
jugate images formed in the object and image spaces respectively by the 
lenses L\ and Z*. These conjugate apertures C'D' and C"D" define the 
apertures which the wave-front must have in the object and image space 
respectively in order to pass through the stop CD or iris of the instrument. 
In the case of several different diaframs in the object space, that diafram for 
which the angle C'MD' in the object region is the least is called the entrance 
FIG. 28. 
pupil of the instrument and the angle 2m'MC' = 2u the angular aperture of 
the instrument; that diafram E'F' (Fig. 29) for which the angle Mm. I in 
the object region is the least, determines the angular field of view and is 
called the entrance field of view diafram or ' 'entrance port"* of instrument. 
Similarly the conjugate image C"D" (Fig. 28) in the image space is called 
the exit or emergence pupil and the corresponding conjugate image E"F" [the 
exit or emergence port. The angle D"M"C" is the projection angle or 
angular aperture in the image space; the angle B"m' A" the image angle 
or angular field of view in the image space. The principal rays are by defi- 
nition those which pass through m, the center of the entrance pupil. From 
Fig. 29 it is evident that, if the entrance port E'F' or object field of view stop 
does not coincide with the object, it cuts off rays from points near margin 
of the object and consequently produces unequal illumination and imper- 
fect correction for the margin of the field. In the microscope it is essential, 
therefore, that the entrance port coincide with the object, and its conjugate 
plane or the exit port with the image plane, in which case the field is sharply 
bounded and equally illuminated throughout. 
Sugfetted by Soutball as translation of The German " F.intrittsluke." 
