CHAPTER IX. 
COLOUR-TREATMENT OF VARIOUS 
OBJECTS. 
HEN we come to classify objects in regard to 
v v their adaptability for photographic treatment, 
we find two prominent characteristics, each presenting 
a difficulty: ist, want of colour or contrast ; 2nd, actual 
or actinic opacity and violence of contrast between 
object and background or between various parts of the 
object. 
Under the first heading come a large number of 
objects pellucid, unstained or lightly stained ; also 
objects stained unsuitably or faded. 
Pellucid objects give the photo-micrographer much 
trouble, especially when the substance composing them 
is of high refracting index. Diatoms having a siliceous 
framework are not much trouble in this respect till we 
get to very high aperture, and then an optical difficulty 
crops up of a nature too abstruse to enter into here. 
Starches, however, unstained yeast, and the like are 
extremely difficult to manage, for it is a crux to get 
contrast between object and ground ; a very useful 
plan, sometimes the only available plan, is to stop out 
