486 MM. C. Lea—TLdentity of the Photosalts of Silver 
KI is exactly the same on the latent image impressed by light 
on AgCOl, AgBr and AgI as it is on the corresponding photo- 
salts. 
IDE 
The impressions produced by alkaline hypophosphite upon 
silver chloride always give rise to direct images, darker than 
the ground on which they are formed. The same substance 
produces an impression on silver bromide which may by devel- 
opment produce either a direct or a reverse image. So that 
there is a perfect parallelism with the action of light. 
The reverse action of light, sometimes called solarization, 
shows itself as follows. A film of bromide being exposed to 
light, part covered, by an opaque screen, the exposed portion 
receives an impression capable of development, and this im- 
pression grows in strength to a certain point, then recedes and 
reaches a condition in which it is less susceptible of develop- 
ment than before exposure. AJ] bromide fi:ms, even unex- 
posed to light, will darken in a developing solution in time. 
The portion that under the action of light has reached the 
reverse stage resists the reducing action of the developer better 
than that which was not exposed at all, and consequently 
appears after development as light on a dark ground and is 
therefore a reverse image. As to the cause of this action we are 
as yet wholly in the dark. If the continued effect of ight was 
simply to restore the affected part to its original state we might 
attempt an explanation by affirming that the continued action 
of light undid its own work. But the fact of the resistance to 
reduction being greater than before exposure shows that some 
as yet unknown action of light is in play. The reverse afetion 
cannot be due to oxidation as has been suggested, because 
hypophosphite reverses and certainly cannot oxidize. 
The reversing action of light on silver bromide finds its 
counterpart in the action of sodium hypophosphite. 
If we take bromide paper (it is immaterial whether in mak- 
ing it the bromide or the silver solution be applied first, but 
for these experiments on the reversal of the image it is essen- 
tial that a pure neutral silver solution be employed and that 
after the second solution has been applied the paper should be 
thoroughly washed, all the operations being of course per- 
formed by inactive light):—if we take such paper and make 
marks on it with a strong solution of hypophosphite, and then 
throw the paper into potassio-ferrous oxalate, we shall get a 
direct development; the marks will be stronger than the 
ground. If now we continually weaken the hy pophosphite 
solution, we shall presently reach a point at which these marks 
are in development almost wholly indistinguishable from the 
