COLOUR-TREATMENT OF VARIOUS OBJECTS. I 07 
result with this class of object. On the whole the best 
way to treat this class of object, which often appears 
impossible with any ordinary “ orthochromatic ” plate, 
is to use the “ Spectrum” plates of Cadett (see p. 98). 
When we come to an object partly dense and non- 
actinic, and partly weak and highly actinic, then, 
indeed, a difficult task is before us. Marked examples 
of this class are insects with deep yellow bodies and 
delicate pale legs; sections with bacteria stained blue 
and tissue red, or even the bacteria red and tissue pale 
blue ; preparations stained briskly with haematoxylin 
and counter-stained with weak eosin or orange; colour¬ 
less bodies in strongly-coloured media, and the like. 
H ere is room for display of skill. For such cases the 
principle to follow is to cut off the pale or non-actinic 
colours so that they may not be impressed on the plate 
till the other details have had time to impress them¬ 
selves. (At the same time it is to be noted that a 
screen, cutting off actinism from the weak parts, is also 
doing the same from the actinically dense parts, which 
increases our difficulty.) 
As instances, Mr. Carnell, who has been singularly 
successful with insect-preparations, uses for them 
yellow-sensitive plates, a yellow—oil lamp—light, and 
a dense yellow screen to boot, giving, of course, great 
exposure. If we have ever really succeeded with blue 
bacteria in red tissue it has been by using a dense 
yellow screen and a plate extra blind to blue. To keep 
back pale eosin in a logwood-stained (nuclear) prepara¬ 
tion, we have used as a screen the solution of Zettnow: 
