To make good blueprints, being guided only by the appearance of the
exposed edge of sensitized paper, requires considerable experience. Very
often, especially on a cloudy day, the edge looks just about right, but
when taken out of the frame and given a rinsing, it is only to find that
the print looks pale because it should have been allowed to remain
exposed for a longer period.
Now simply take a small test-piece of the same paper (say about 4 inches
square) and a piece of tracing cloth with several lines on its surface
and lay these small pieces out at the same time the real print is being
exposed, and cover these samples with a piece of glass about 4 inches
square. As a general rule, we can find a place on top of the frame for
the testing-piece, and by having a small dish of water at hand for
testing the print by tearing off a small bit and washing same to note
its appearance, the novice can get just as good results as the
experienced hand without danger of failure.