115 
It is well known that when plates are over exposed in 
the camera the action of light becomes reversed ; thus, a sky 
which ought to be black becomes transparent, and a print 
from this looks somewhat like a negative on paper. Still 
the action is by no means regular, and there are many points 
connected with this action which require careful investiga- 
tion. 
It is my intention to bring some of these before you this 
evening in a brief manner, and to exhibit some curious speci- 
mens in illustration. 
All photographers must have noticed now and then where 
two lines of light and dark meet, that the light is rendered 
more intense just at the edge of the black or dark object ; 
in portraits of gentlemen this is sometimes very prominent, 
the dark figure is thrown up as intended from the light 
background, but the latter is so much more intense just 
round the figure that it has sometimes the appearance of 
light streaming from behind. This effect is sometimes 
seen in landscapes — round a tower or even a tree pro- 
jected against the sky runs a line of intense white much 
deeper than the sky itself. On the other hand it sometimes 
happens that in a landscape with mountains the edge against 
the sky seems to be robbed as it were of a portion of its 
intensity, and so when printed exhibits a dark edge. 
Again, it is well known that the sky, seen through small 
openings in trees or small windows in a ruin, is much more 
intense in a negative than the great mass of the sky ; also 
that in taking an interior view, where light streams through 
a window in front of the camera, the effect produced on the 
negative is as if the light came round, illuminating to some 
extent the inside, and gradually as it were eating away the 
tracery of the window. 
In some cases where the bright sky is seen through an 
opening of the trees, the trees and branches around the 
opening are converted in the photograph into pure white 
