with the material of the latent Photographic Image. 487 
ground on which they were made. But continuing the dilu- 
tion still further we presently reach a point at which the marks 
reappear, but this time reversely ; as lighter marks on a darker 
sround. This result is obtained with about a two per cent 
solution; the first mentioned effect comes with a solution of 
25 or 30 per cent. So that according as we use the hypophos- 
phite solution stronger or weaker we obtain exactly opposite 
effects. Here the parallelism is striking but not perfect, at 
least it remains to be explained why the action in the two cases 
proceeds in a reverse order. All the other reactions show a 
perfect identity. 
Time will also sometimes produce the same effect as dilution. 
Paper marked and put away for 24 or 48 hours, giving at first 
a direct image may after that time give a reverse one. This 
effect is extremely uncertain and I think, exceptional. I have 
kept very many pieces for periods from a few hours to several 
weeks, which first and last gave direct images only; a few 
became reversed. But the experiment itself, the opposite 
effect of strong and weak solutions, is liable to no such uncer- 
tainty. 
Again, we may make light and hypophosphite interact and 
each reverse the other’s action. 
To make hypophosphite reverse the action of light, I take a 
piece of silver bromide paper, expose it to the action of diffuse 
daylight for a few seconds, then taking it to the dark room, 
make marks upon it with a glass rod dipped in solution of 
hypophosphite. On developing with potassio-ferrous oxalate 
the marks appear lighter than the ground. Or what is perhaps 
more striking, we take two pieces of such paper, retain one in 
the dark room and expose the other from 5 to 20 seconds to 
diffuse light, then make marks on both with a glass rod dipped 
in strong solution of hypophosphite, and then, after allowing a 
few minutes for the hypophosphite to act, place them both in a 
solution of potassio-ferrous oxalate. The marks will develop, 
in the one case, as dark on a light ground, in the other (that 
exposed to daylight) as hght on a dark ground. | 
We have here made the hypophosphite imitate the action of 
light: it has reversed the image in the same way as would 
result from a prolonged exposure to light. It will next be 
shown that light may be made to imitate the action of hypo- 
phosphite and reverse the effect already produced by that or 
other reducing agent. 
Tf we take the red or purple silver bromide, preparing it-with 
exclusion of light and the same precaution as in the case of a 
dry plate and extend it over paper (it is best though not essen- 
tial, to mix it with a little gelatine to enable it to retain its 
hold on the paper in the subsequent treatment) dry it and 
