g8 
PRACTICAL PHOTO-MICROGRAPHY. 
and the “ medium.” The latter are more useful gene¬ 
rally for photo-micrography, as with them it is easy to 
get good density and contrast. But for objects pre¬ 
senting in themselves great contrast the instantaneous 
plates may sometimes be preferable. 
The plates most commonly used of late years by the 
writer are made by Messrs. Lumiere, of Lyons, who 
have now a branch in London. The special brand used 
is known as “Series A” and are “sensitive to yellow and 
green.” We obtain them as “slow” as possible, in 
order to have great density and clearness at command, 
and we develop them with the hydrokinone and caustic 
solution given later. As a rule we use these with a 
yellow screen of such a depth as to increase the 
necessary exposure about three times as compared with 
no screening, yet the average exposure at 1,000 diameters 
with limelight is only about twenty-five seconds. For 
work where we require dense high lights and clear, black¬ 
printing shadows, such as bacteria on clean grounds and 
diatoms, we know no plates to beat these. With full 
exposure and weaker development they answer very 
well for lower power work and tissue-sections, and 
the like. 
But when we have to deal with really difficult problems 
in colour-rendering, we have recourse to the “Spectrum” 
plates of Cadett and Neall, and of these we choose a 
brand of medium sensitiveness, or even in some cases 
the “ Slow ” quality. These remarkable plates are 
more or less sensitive to all visible parts of the spectrum, 
and consequently the greatest care is required in 
