produce it is reduced to a minimum. It will also, I think, be evident
that a uniform pressure, of any desired intensity, can be had all over
the surface of the sensitized paper for the purpose of securing perfect
contact between it and the negative. The blue copies that are taken with
this apparatus are entirely free from blue lines when the negatives,
chemicals, and paper are good.

_The Mechanism for Adjusting the Surface of the Glass, until it shall be
Perpendicular to the Direction of the Sun's Rays._--I have found many
uses for the blue copying process in connection with the work of
instruction at the Massachusetts Institute of Technology. Notes printed
by it are far better and less costly than those printed by papyrograph.
I will not detain you now with an account of the uses that I have made
of it. I will merely say that more than a year ago I found that my
frame, which has a glass 3 feet x 4 feet, was wholly inadequate to the
work in hand, and I tried to increase the production from it by
diminishing the time of printing. The glass of this frame was
horizontal, except when one of its ends was tilted off from the slides
which guided it when pushed out of the window; and I knew that it took
three or four times as long to print when the sun was low as it did when
the sun was near the meridian. I made plans for mounting this frame upon
a single axis, about which it could be turned after it had been pushed
through the window, but I saw that no movement about a single axis would
give a satisfactory adjustment for all times of the year, and I
considered what arrangement of two axes would permit a rapid and perfect
adjustment, at all times, with the least trouble to the operator. It was
evident that when the sun was in the equatorial plane, the surface of
the glass should contain a line which was parallel to the axis of the
earth; and further, that if such a glass was firmly attached to an axis
which was parallel to that of the earth, it would fulfill the desired