lm. Sir Wm. Abney has shown
that these sensitisers are active even if laid on as a varnish on
the sensitive surface and removed before development. It must be
remembered, however, that at temperatures of about -50 deg. these
sensitisers lose much of their influence on the film; as I have
pointed out in a paper read before the Photographic Convention of
1894.

It. appears to me that on these views the curious phenomenon of
recurrent reversals does not present a problem hopeless of
explanation. The process of photo-

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ionisation constituting the latent image, where the ion is
probably not immediately neutralised by chemical combination,
presents features akin to the charging of a capacity--say a Leyden
jar. There may be a rising potential between the groups of ions
until ultimately a point is attained when there is a spontaneous
neutralisation. I may observe that the phenomena of reversal
appear to indicate that the change in the silver bromide
molecule, whatever be its nature, is one of gradually increasing
intensity, and finally attains a maximum when a return to the
original condition occurs. The maximum is the point of most
intense developable image. It is probable that the sensitiser--in
this case the gelatin in which the bromide of silver is
immersed--plays a part in the conditions of stability which are
involved.

Of great interest in all our considerations and theories is the
recent work of Wood on photographic reversal. The result of this
work is--as I take it--to show that the stability of the latent
image may be very various according to the mode of its formation.
Thus it appears that the sort of latent effect which is produced
by pressure or friction is the least stable of any. This may be
reversed or wiped out by the application of any other known form
of photographic stimulus. Thus an exposure to X-rays will
obliterate it, or a very brief exposure to light. The latent
image arising from X-rays is next in order of increasing
stability. Light action will remove this. Third in order is a
very brief light-shock or sudden flash. This

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cannot be reversed by any of the foregoing modes of stimulation,
but a long-continued undulatory stimulus, as from lamp-light,
will reverse it. Last and most stable of all is the gradually
built-up configuration due to long-continued light exposure. This
can only be reversed by overdoing it according to the known facts
of recurrent reversal. Wood takes occasion