FIXING, INTENSIFICATION, ETC., OF NEGATIVE. 85 
Extraneous objects in the field, or objects in the 
background which are out of focus. 
Movement of objects,in fluid mounts, during time 
of exposure. 
Dirt upon the slide behind the object, upon 
the cover-glass, or upon any part of the optical 
apparatus. 
Imperfect focusing ; movement of apparatus during 
time of exposure, due to vibration of building 
caused by the wind, passing trains, etc. 
Over-exposure produces weak and foggy image. 
Under-exposure produces strong and clear shad- 
ows. 
Under-development produces weak image and 
clear shadows. 
Dust or air-bubbles on the plate produce transparent 
spots. 
Hyposulphite of soda not well washed out causes 
an appearance of crystallization when the nega- 
tive is dry. 
Too much iron in the developer causes a yellow 
gritty precipitate upon the plate. 
When a negative is defective from any of these causes, 
TRY AGAIN. 
Owing to the many causes of failure and the 
difficulties of technique already pointed out, a good 
negative is a valuable piece of property, and should be 
carefully preserved. 
Those which are to be retained, when thor- 
oughly dry, are to be varnished with negative 
varnish. Mountfort’s crystal varnish has a good 
reputation, and this or any other varnish recom- 
