bly get it;
thick inks, such as Indian, require coarser pens.

The sharp edge is taken off and the width of the point reduced by drawing
the pen at an angle along the stone, revolving it all the time. The nearer
to the hole you can wear the glass away the finer will be the line made by
the pen.

Another method is as follows:--Seal the point by holding it a moment in the
flame. A tiny bulb forms on the end, and this has to be ground away till
the central hole is reached. This is ascertained by the water test, or by
holding the pen point upwards, so that light is reflected from the tip, and
examining it under the magnifier. Then grind the edge off, as in the first
case.

Care of Pens.--The ink should be well strained, to remove the smallest
particles of "suspended matter," and be kept corked. Fill the pen by
suction. On no account allow the ink to dry in the pen. Squirt any ink out
of it when it is done with, and place it point downwards in a vessel of
water, which should have a soft rubber pad at the bottom, and be kept
covered to exclude dust. Or the pen may be cleaned out with water and
slipped into a holder made by rolling up a piece of corrugated
packing-paper. If the point gets stopped up, stand the pen in nitric or
sulphuric acid, which will probably dissolve the obstruction; and
afterwards wash it out.

Inks.--I have found Stephens's coloured inks very satisfactory, and can
recommend them.

Paper and Cards.--The paper or cards used to draw the figures on should
not have a coated surface, as the coating tends to clog the pen. The
cheapest suitable material is hot pressed paper, a few penny-worths of
which will suffice for many designs. Plain white cards with a good surface
can be bought for from 8s. to 10s. per thousand.

Lantern Slides.--Moisten one side of a clean lantern slide plate with
paraffin and hold it over a candle flame till it is a dead black all over.
Very fine tracings can be obtained on the smoked surface if a fine steel
point is substituted for the glass pen. The design should be protected by a
cover-glass attached to it by a binding strip round the edges.

Details of Harmonographs.

The reader may be interested in details of the apparatus shown in Figs. 168
and 170, made by the writer.

The Rectilinear Harmonograph, shown in Fig. 168, has pendulums of 5/8-inch
wood, 40 inches long, suspended 30 inches from the lower ends, and set 10
inches apart, centre to centre. The suspensions are