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4 RA }'S OF POSITIVE ELECTRICITY

aid for studying the way the rays are deflected by
arid magnetic forces, and it is important to find the
which gives the brightest phosphorescence. The
which ! have found most useful is wlllemite, a natural
of zinc. The mineral should be ground into as fine a
as possible, the powder shaken up In alcohol so as to
form a suspension, which is allowed to deposit slowly on a
plate; by this means the glass is covered with an exceed-
ingly even deposit of the willemite. After continued exposure
to the positive rays the brightness of the phosphorescence
and ultimately disappears, so that for the detection
of rays the willemite must be renewed from time to time.

substances deteriorate more rapidly than others, for ex-
ample zinc blende phosphoresces very brightly under the
positive rays, but, as far as my experience goes, it deteriorates
more quickly than willemite, so that when the observa-
tions have to last for any considerable time the willemite is
preferable. A more sensitive, and for many purposes more
convenient, way of registering the deflection of the positive
rays is to take advantage of the fact that, when these rays
strike against a photographic plate, they affect the plate at the
place of impact and thus a permanent record of the position
of the rays can be obtained. The action of the rays on the
plate differs from that of light, since these rays do not use the
whole thickness of the film but only a layer close to the surface,
so that it does not follow that the most " rapid " photographic
plates are the most sensitive to the positive rays. The most
sensitive plates for the detection of the positive rays would be
having very thin films containing as much silver as pos-
sible. I have tried the old Daguerreotype process instead of
the usual dry plate method, but without much success. It is
probable that if Schumann plates (Baly's " Spectroscopy," p.
359) could be prepared as uniform and free from streaks as



	4 RA }'S OF POSITIVE ELECTRICITY aid for studying the way the rays are deflected by arid magnetic forces, and it is important to find the which gives the brightest phosphorescence. The which ! have found most useful is wlllemite, a natural of zinc. The mineral should be ground into as fine a as possible, the powder shaken up In alcohol so as to form a suspension, which is allowed to deposit slowly on a plate; by this means the glass is covered with an exceed- ingly even deposit of the willemite. After continued exposure to the positive rays the brightness of the phosphorescence and ultimately disappears, so that for the detection of rays the willemite must be renewed from time to time. substances deteriorate more rapidly than others, for ex- ample zinc blende phosphoresces very brightly under the positive rays, but, as far as my experience goes, it deteriorates more quickly than willemite, so that when the observa- tions have to last for any considerable time the willemite is preferable. A more sensitive, and for many purposes more convenient, way of registering the deflection of the positive rays is to take advantage of the fact that, when these rays strike against a photographic plate, they affect the plate at the place of impact and thus a permanent record of the position of the rays can be obtained. The action of the rays on the plate differs from that of light, since these rays do not use the whole thickness of the film but only a layer close to the surface, so that it does not follow that the most " rapid " photographic plates are the most sensitive to the positive rays. The most sensitive plates for the detection of the positive rays would be having very thin films containing as much silver as pos- sible. I have tried the old Daguerreotype process instead of the usual dry plate method, but without much success. It is probable that if Schumann plates (Baly's " Spectroscopy," p. 359) could be prepared as uniform and free from streaks as