ll directions, so that the
pen point may rest squarely on the card. (Fig. 175.)

Another method is to attach to the end of the lever a vertical half-tube
of tin, against which the pen is pressed by small rubber bands; but even
more convenient is a small spring clip shaped as in Fig. 176.

[Illustration: FIG. 176.--Clip to hold glass pen.]

The card platform should be perfectly flat. This is essential for the
production of good diagrams. If wood is used, it is advisable to glue two
thin pieces together under pressure, with the grain of one running at right
angles to the other, to prevent warping.

Another important point is to have the card platform square to the rod. If
a piece of tubing fitting the rod is turned up true in the lathe and
soldered to a disc screwed to the underside of the table, perpendicularity
will be assured, and incidentally the table is rendered detachable.

To hold the card in place on the table, slit a spring of an old
photographic printing frame down the middle, and screw the two halves,
convex side upwards, by one end near two opposite corners of the platform.
(See Fig. 170.) If cards of the same size are always used, the table should
be marked to assist adjustment.

Making Pens.--The most satisfactory form of pen is undoubtedly a piece of
glass tubing drawn out to a point, which is ground down quite smooth. The
making of such pens is rather a tedious business, but if care be taken of
the pen when made it will last an indefinite time.

Tubing 3/16 or 1/8 inch in external diameter is suitable. Break it up (by
nicking with a file) into 9-inch lengths. Take a piece and hold its centre
in the flame of a small spirit lamp, and revolve it till it softens. Then
draw the glass out in as straight a line as possible, so that the points
may be central. If the drawing is done too fast, the points will be much
too long to be of any use: half an inch of taper is quite enough.

Assuming that a point of satisfactory shape has been attained--and one
must expect some failures before this happens--the pen may be placed in the
pen lever and ground down on a perfectly clean wet hone laid on the card
platform, which should be given a circular movement. Weight the lever so as
to put a fair pressure on the point.

The point should be examined from time to time under a strong
magnifying-glass, and tested by blowing through it into a glass of water.
For very liquid ink the hole should be as small as you can possi