IOO 
PRACTICAL PHOTO-MICROGRAPHY. 
colour and of the same refraction-index as glass, this 
reflection is suppressed or at least minimised, to the 
great advantage of the definition of the image. We do 
not find this defect, called by photographers “halation,” 
to any great degree in the Lumiere plates, but with 
very sensitive plates such as the “ Spectrums ” we 
always employ backing. Certain sticky plasters faced 
with suitable material are to be found on the market, 
and one of these may be rubbed on to the back of each 
plate before exposure and removed before development, 
each plaster in this way serving as a backing for many 
plates. 
To render ordinary plates orthochromatic we proceed 
thus. The plates are bathed in the following solution 
for two minutes, and then dried in a drying press such 
as is used by photographers : 
Solution of erythrosin (i in r,ooo) ... ... i part 
Dilution of liq. amnion, fort, (i to 10) .. i ,, 
Water to ... ... ... ... ... ... io parts 
The erythrosin is the sodium or potassium salt of 
•tetraiodofluorescein, and is made by the Badischer 
Anilin und Soda Fabrik. In solution it is not fluo¬ 
rescent. 
The dye, cyanin, renders plates sensitive to even red 
rays, which erythrosin does not, and cyanin may be 
used ; but in this case the work has to be carried on in 
almost complete darkness, though some of the red light 
may be cut off by green glass or other medium. Even 
for yellow sensitive plates great care must be taken to 
work in a very subdued ruby light, and not to expose 
plates more than necessary to that. 
