8.2.
OBJECTIVE APERTURE POSITION AND SIZE
269
reasonably well equipped with such apertures, and the investigator
accepts them as part of the basic instrument without giving them further
thought.
Small apertures are now also used in the condenser lens, partly to increase the contrast, but here they have at least one additional important
function. "Space charge" effects occur if electrons are crowded too closely
together. This can easily happen as an intense beam passes through a
focal point. If this should occur in the objective lens near beam "crossover" it destroys resolution. This is avoided by limiting the possible beam
intensity by using a condenser aperture of suitable size. Some modern
microscopes also refine the beam considerably before it reaches the
specimen by using more than one condenser lens.
In the subsequent portions of the chapter no attempt will be made to
consider how particular microscopes should be handled or maintained.
These details naturally vary much from one manufacturer's product to
another, and this sort of instruction and maintenance is available from
their handbooks and service departments. On the other hand, as these
introductory remarks have indicated, those of us working with tissues
have special problems which do need understanding and detailed consideration. These are in addition to certain of the general problems of
maintenance which every microscopist must master in a practical way.
To operate a microscope successfully and run a laboratory, the investigator must be able to take care of the microscope column himself. He
cannot rely on the manufacturer's service department to do this for it
requires day to day watchfulness, and minor adjustment. The operator
must know the warning signs of impending difficulty. Usually he will be
able to correct it himself and at least save time, but also he must know
when he needs help and when to call a service man, engineer, or physicist.
Thus, a portion of this chapter is devoted to commonplace problems of
maintenance without reference to particular types of instruments.
8.2. Objective Aperture Position and Size
There are two places where a small aperture can be placed in an
objective pole-piece to screen out effectively most of the scattered electrons. It may either be between the two lens elements of the pole-piece
or be in the back focal plane. In the between-the-lens position the cross-
OBJECTIVE APERTURE POSITION AND SIZE
269
reasonably well equipped with such apertures, and the investigator
accepts them as part of the basic instrument without giving them further
thought.
Small apertures are now also used in the condenser lens, partly to increase the contrast, but here they have at least one additional important
function. "Space charge" effects occur if electrons are crowded too closely
together. This can easily happen as an intense beam passes through a
focal point. If this should occur in the objective lens near beam "crossover" it destroys resolution. This is avoided by limiting the possible beam
intensity by using a condenser aperture of suitable size. Some modern
microscopes also refine the beam considerably before it reaches the
specimen by using more than one condenser lens.
In the subsequent portions of the chapter no attempt will be made to
consider how particular microscopes should be handled or maintained.
These details naturally vary much from one manufacturer's product to
another, and this sort of instruction and maintenance is available from
their handbooks and service departments. On the other hand, as these
introductory remarks have indicated, those of us working with tissues
have special problems which do need understanding and detailed consideration. These are in addition to certain of the general problems of
maintenance which every microscopist must master in a practical way.
To operate a microscope successfully and run a laboratory, the investigator must be able to take care of the microscope column himself. He
cannot rely on the manufacturer's service department to do this for it
requires day to day watchfulness, and minor adjustment. The operator
must know the warning signs of impending difficulty. Usually he will be
able to correct it himself and at least save time, but also he must know
when he needs help and when to call a service man, engineer, or physicist.
Thus, a portion of this chapter is devoted to commonplace problems of
maintenance without reference to particular types of instruments.
8.2. Objective Aperture Position and Size
There are two places where a small aperture can be placed in an
objective pole-piece to screen out effectively most of the scattered electrons. It may either be between the two lens elements of the pole-piece
or be in the back focal plane. In the between-the-lens position the cross-