while a dark
portion would be made up of wide lines close together; a black part would
appear solid and show no lines at all. It is, of course, obvious {116} that
the lines on the negative cannot be wider apart, centre to centre, than the
lines of the screen. A good screen distance has been found to be 1 to 64,
_i.e._ the diameter of the stop is 1/64th of the camera extension, and the
distance of the screen lines from the photographic plate is 64 times the
size of the screen opening. The following table shows what this distance is
for the screen most likely to be used. The line screens used consist of
glass plates upon which a number of lines are accurately ruled, the width
of the lines and the spaces between being equal; the lines are filled in
with an opaque substance. These ruled screens are very expensive, and are
only made to order,[10] a screen half-plate size costing from 21s. to 27s.
6d. An efficient substitute for a ruled screen can be made by taking a
rather large sheet of Bristol board and ruling lines across in pure black
drawing ink, the width of the lines and the spaces between being 1/12th of
an inch respectively. A photograph must be taken of this card, the
reduction in size determining the number of lines to the inch. A card 20 x
15 inches, with 12 lines to the inch, would, if reduced to 5 x 4 inches,
make a screen having 48 lines to the inch. Preparing the board is rather a
tedious operation, but the line negative produced will be found to give
results almost as good as those obtained from a purchased screen.

DIAMETER OF STOP USED 1/64TH OF CAMERA EXTENSION.

--------------------------------------------------------------
|Screen ruling |Actual space| Distance of |In 1/32|In milli-|
|lines per inch.| in inches. |screen ruling| inches| metres.|
| | | in inches. | | |
|---------------+------------+-------------+-------+---------|
| 35 | 1/70 | .91 | 28.8 | 21.8 |
| 50 | 1/100 | .64 | 20.5 | 16.2 |
| 65 | 1/130 | .49 | 15.7 | 12.4 |
--------------------------------------------------------------

As it is impossible for many to have the use of professional apparatus
designed for this particular kind of work, {117} the fixing of the screen
into an ordinary camera must be left to the ingenuity of the worker. A
half-plate back focussing camera will be found suitabl