io6 
PRACTICAL PHOTO-MICROGRAPHY. 
With pale stained, or faded, objects contrast may to 
a certain extent be secured by stopping down, but, as 
before, it is always at the expense of definition. Under 
this head come a great number of preparations, made 
by those who are not aware of the desiderata for photo¬ 
graphy ; we have sections stained poorly with violet 
logwood, bacteria with weak gentian or methyl 
violet, and with over-washed methyl blue. When we 
have any of these dyes properly used there is little 
trouble ; and in our last chapter attention will be called 
to this matter of staining suitably for photography. 
The principle to be borne in mind is, to use a plate 
comparatively insensitive to the stain-colour, and a 
screen cutting off more or less of that colour. 
On the other hand, when we have to treat objects of 
great density, or heavily stained with some non-actinic 
dye, we must follow other courses. The best example 
of this kind of object is that of the usual insect-prepar¬ 
ations, and of thick sections which are all too common. 
The treatment of such objects consists chiefly of pro¬ 
longed exposure, and the omission of everything which 
will cut off rays of the colour of the object, as well as 
the use of plates highly sensitive to the colour. Thus a 
very yellow insect may be tried with a yellow-sensitive 
plate, without any screen ; a thick logwood preparation 
may be tackled with an ordinary plate, or, if the lens is 
achromatic, an orthochromatic plate, possibly with a 
blue screen to keep back the ground till the object is 
impressed on the plate. In practice, however, pro¬ 
longed exposure alone usually gives the best available 
