THE APPARATUS
ings are on both sides of the box and about 5 cm. in dia-
meter. When the silk thread is wound up the strip DEFG of
meter. When the silk thread is wound up the strip DEFG of
photographic plate in the
box Is above the opening
A, so that there Is a free
way for the rays to pass
through A and fall on a
willemite screen behind
It This screen is not
used for purposes of
measurement, but only
to see before taking the
photograph that the tube
Is giving an • adequate
supply of positive rays.
The box Is sufficiently
large to hold a film long
enough for two or more
photographs ; If It Is
wished to take two
photographs, the plate
Is lowered until the bot-
tom half comes opposite
to the opening A, a
photograph Is taken In
this position, the plate
Is then let down still
further until the top half
of the plate comes oppo-
box Is above the opening
A, so that there Is a free
way for the rays to pass
through A and fall on a
willemite screen behind
It This screen is not
used for purposes of
measurement, but only
to see before taking the
photograph that the tube
Is giving an • adequate
supply of positive rays.
The box Is sufficiently
large to hold a film long
enough for two or more
photographs ; If It Is
wished to take two
photographs, the plate
Is lowered until the bot-
tom half comes opposite
to the opening A, a
photograph Is taken In
this position, the plate
Is then let down still
further until the top half
of the plate comes oppo-
site to the opening, then
FIG. 12.
a second photograph Is
taken. This plan Is convenient because the deflections of
the different kinds of positive rays differ so much that it is
taken. This plan Is convenient because the deflections of
the different kinds of positive rays differ so much that it is