ORTHOCHROMATIC PHOTOGRAPHY BS) 
dyes are very tenaciously retained by the gelatine and can 
only be discharged by a nitrite bath, such as: 
Sodium nitrite. lg 8 er. 
Hydrochloric acid 10 ccm 80 min. 
Water 1000 ccm 16 oz. 
or a 2 per cent alum solution with an equal volume of hydro- 
chloric acid. The Hoechst dye works have introduced two 
new dyes for the same purpose, pinakryptol and pinakryptol 
green; the latter is the more soluble of the two and may be 
used as a preliminary bath or added to the developer, but 
the strength should be only 1:5000. Neither of these dyes 
stain the gelatine so much as the safranines. Lumiére and 
Seyewetz have found that aurantia in a 1: 1000 solution acts 
as a good desensitiser, and also toluylene red. This desensi- 
tising process is also applicable to screen-plates. 
ORTHOCHROMATIC FILTERS.—Yellow filters are used to 
cut down the excessive blue and violet sensitiveness of colour- 
sensitised plates. The quantity of dye given is calculated in 
grams of the dry dye per square meter, or grains per 1000 
square inches. It is advisable to dissolve the dyes in distilled 
water and add to the gelatine solution, previously prepared 
and filtered. 
To make the gelatine solution, immerse 125 g (614 gr.) 
soft gelatine in 500 ccm (8 oz.) distilled water, stir for 
5 minutes, and pour off the water. Again add the same 
quantity of water, allow the gelatine to soak for 10 minutes 
with an occasional stir, and pour off. Repeat the operation, 
allowing the gelatine to soak for 15 minutes, and pour off 
the water. Drain out as much water as possible, then melt 
the gelatine on a water bath in the water that it has absorbed. 
Add sufficient distilled water to make the total bulk 1000 ccm 
(16 oz.), and filter through glass wool or well- oe tit 
absorbent cotton. Qs! 
