5 inches in diameter. The lead screw has 30 threads to the inch,
and the reduction between it and the drum is 3:1, so that the table has a
movement of 1/90th inch per revolution of the drum.

From the brief description of the various types of machines that have been
given it will be apparent that in the design of the machine proper there is
nothing very complicated, although the addition of the driving and
synchronising apparatus complicates matters rather considerably. The
questions of driving and synchronising the machines at the two stations is
fully dealt with in Chapter IV.

[Illustration: FIG. 10a.]

[Illustration: FIG. 10b. Enlarged view of an image broken up by a cross
screen.]

{21} Although the design of the machines is rather simple great attention
must be paid both to accuracy of construction and accuracy of working, and
this applies, not only to the machines (whether for transmitting or
receiving) but for all the various pieces of apparatus that are used. Too
much care cannot be bestowed upon this point, as in the wireless
transmission of photographs there is a large number of instruments all
requiring careful adjustment, and which have to work together in perfect
unison at a high speed.

The machine shown in Figs. 10 and 10_a_ was designed and used by the writer
solely for experimental work. It will be noticed in the description given
in the appendix of the method of preparing the metal prints that a 5" x 4"
camera is recommended, while the machine, Fig. 10, is designed to take a
print procured from a quarter-plate negative. This size of drum was adopted
for several reasons, and although it will be found quite large enough for
general experimental work the writer has come to the conclusion that for
practical commercial work a drum to take a print 5" x 4" will give better
results.

In making a negative of a picture that is required for reproduction
purposes, the line screen in the camera is replaced by a "cross screen,"
_i.e._ two single line screens placed with their lines at an angle of 90deg
to one another, and this breaks the {22} image up into small squares
instead of lines. By looking at any ordinary newspaper or book illustration
through a powerful magnifying glass the effects of a cross screen will
readily be seen. With a cross screen a certain amount of detail is
necessarily lost, but with a single line screen the amount lost is much
greater. If there is any very small detail in the picture m