(5) Tracing screen images
Comparison of two similarly shaped specimens is conveniently parformed byoverlay-
ing the screen image of one specimen traced on translucent paper upon the pro-
jected image of the other on the screen, or by placing both traced images of speci-
mens upon the other. If necessary, the traced images on the paper can be kept
for later use.
As paper is breakable and liable to expansion and contraction, it is not suitable for
accurate work and for use after a long period.
Sheet of plastic in place of paper may be better, though more expensive.
Plastic sheet with a mat surface on which fiducial lines or figures can be drawn
directly is available and also used as a master chart as described previously.
Fig. 27
focusing glass and a plate holder in which
When using the photographing screen, no
To take photograph, make the room
dark, hold a sheet of photosensitive
plate, film, paper, etc. on the screen
and give an appropriate exposure by
switching on and off the illuminator of
the projector.
There is a photographing screen,
available on order which is mounted
on the instrument in place of the
normal screen. It is provided with a
plate, film or paper is put beforehand.
dark room will be necessary.
No correct exposure time can be given here, because they vary with the surface of
specimen, type of illumination, magnification, voltage of the lamp being used, etc.
The surest way to determine the correct exposure is to expose a test plate in steps
of time and giving the best result. If the exposure time is too short to be controlled
in such a case where a high sensitive material is used, it will be better to decrease
the intensity of the lamp, or it will be convenient also to put in and out a piece of
cardboard underneath the projection 'lens, instead of switching on and off the lamp.
For accurate measurement, photographic plate is preferably used to paper as the
former is less liable to expansion or shrinkage.
The photograph of projected image
taken as a permanent record has an
advantage that it permits examination
and measurement at a later time.
(6) Photographing screen image (Fig. 27)
- 16 -