COLOUR-TREATMENT OF VARIOUS OBJECTS. 105 
the central pencil of light by putting into the condenser 
one of the central opaque discs shown in fig. 14. 
There is a great temptation in dealing with this kind 
of object to shut down the condenser to such a point 
that there appears to be contrast ; but this leads to 
fuzzy diffraction images, and should, so far as possible, 
be avoided. With pellucid and very lightly stained 
objects the contrast will always be increased by stopping 
down the condenser, but in so doing we are impairing 
definition, and we should make a point of stopping 
down just as little as jiossible. But it is admitted that 
sometimes a considerable amount of ajuerture must be 
cut off to obtain anything like contrast. The central 
stoji is decidedly the best help in such conditions. 
If the object is slightly stained with any colour what¬ 
ever we have to meet the difficulty by use of colour, 
correct plates helped out with a screen. Broadly, the 
line to follow is to use a plate little sensitive to the 
. stain colour, and to use a screen cutting off the colour 
of the preparation. Thus, if the weak stain is violet, as 
logwood, or pale blue, as methyl blue, our plan would 
be to use a yellow-sensitive jdate and a strong yellow 
screen. If the stain is weak yellow, as Bismarck brown, 
01 red, as some eosins, we may use an ordinary jdate, 
with an apochromatic or corrected lens, or an ortho- 
chromatic erythrosin—-plate, in either case with a dark 
blue or green screen. Some of the eosm stains are 
very deceptive ; they appear red or pink to the eye, but 
are found to transmit much blue. For these we find a 
green screen with an erythrosin plate to answer well. 
H 
