pressure; this is very
likely the case. Whether this effect is a direct function of the
density of the gas in the tube, or whether it is dependent on the
voltage or time integral of the current during the discharge, are
questions which still await a solution.

The preparation of calcium tungstate for fluorescent screens is very
simple.

Commercial sodium tungstate is fused with dried calcium chloride in
the proportion of three parts of the former to two parts of the
latter, both constituents being in fine powder and well mixed
together. The fusion is conducted in a Fletcher's crucible furnace in
a clay crucible. The temperature is raised as rapidly as possible to
the highest point which the furnace will attain--i.e. a pure white
heat. At this temperature the mixture of salts becomes partly fluid,
or at least pasty, and the temperature may be kept at its highest
point for, say, a quarter of an hour. At the end of this time the
mass is poured and scraped on to a brick, and when cold is broken up
and boiled with a large excess of water to dissolve out all soluble
matter. The insoluble part, which consists of a gray shining powder,
is washed several times with hot water, and is finally dried on
filter paper in a water oven.

In order to prepare a screen the powder is ground slightly with very
dilute shellac varnish, and is then floated over a glass plate so as
to get an even covering. Unless the covering be very even the screen
is useless, and no pains should be spared to secure evenness. It is
not exactly easy to get a regular coat of the fluorescent material,
but it may be done with a little care.

CHAPTER II

GLASS-GRINDING AND OPTICIANS' WORK

Sec. 52. As no instructions of any practical value in this art have, so
far as I know, appeared in any book in English, though a great deal of
valuable information has been given in the English Mechanic and
elsewhere, I shall deal with the matter sufficiently fully for all
practical purposes. On the other hand, I do not propose to treat of
all the methods which have been proposed, but only those requisite for
the production of the results claimed. The student is requested to
read through the chapter before commencing any particular operation.

Sec. 53. The simplest way will be to describe the process of manufacture
of some standard optical appliance, from which a general idea of the
nature of the operations will be obtained. After this preliminary
account