g or enlarging, only in this case the camera
should be a fixture and the board, A, arranged to slide backwards and
forwards instead.

[Illustration: FIG. 61.

Portions of photographs (full size) of single line screen, and single line
print. Screen 40 lines to the inch.]

[Illustration: FIG. 62.]

{125} An extra improvement would be to rule the surface of the copying
board, A, in a manner similar to that shown in the diagram, Fig. 63. The
rulings should be marked off from the centre of the board, and should
enclose parallelograms of the various plate sizes ranging from 3-1/4 x
4-1/4 inches up to the full size of the board. By fastening the picture or
photograph to be copied in the space on the board corresponding in size, we
can ensure that it is in the correct position for the whole to be included
on the photographic plate, providing, of course, that the centre of lens
and board coincide.

With regard to the lens required, the practice adhered to by most
photographers is to use a lens having a focal length equal to the diagonal
of the plate used. Thus for a 1/4-plate camera a 5-inch lens should be
used, and for a 1/2-plate an 8-inch lens, and so on. For a 5 x 4 inch
camera a 6-inch lens will be required. The following is a simple rule for
finding the conjugate foci of a lens, and is useful in obtaining the
distance from the lens to the photographic plate and the picture to be
copied. Let us suppose that we wish to make a 1-1/2 times enlarged line
negative from a 4-1/4 x 3-1/4 inch print. Add 1 to the number of times it
is required to enlarge and multiply the result by the focal length of the
lens in inches. In the present case this will be 1-1/2 + 1 = 2-1/2; and if
a 6-inch lens is used, 2-1/2 x 6 = 15 inches will be the distance of the
lens from the plate. Divide this number by the number of times it is
desired to enlarge, and the distance of the lens from the picture to be
copied is obtained; in this instance 15 / 1-1/2 = 10 inches. The same rule
can be followed when it is required to reduce any given number of times,
only in this case the greater number will represent the distance between
the lens and the picture to be copied, and the lesser number the distance
between the lens and the plate.

In reducing, a 1/4-plate lens will be found to fully cover a 5 x 4 inch
plate, providing the reduction is not greater than three to one.

* * * * *

{126}

APPENDIX C

LENSES

In