MAPPING THE LEAST REFRANGIBLE END OF THE SOLAR SPECTRUM. 657 
is most important that the final washing should be conducted almost in darkness. It 
is also essential to eliminate all traces of nitric acid, as it retards the action of light on 
the bromide, and may destroy it if present in any appreciable quantities. To prepare 
the plate with this silver bromide emulsion all that is necessary is to pour it over a 
clean glass plate, as in ordinary photographic processes, and to allow it to dry in a 
dark cupboard.* 
For development after exposure I recommend what is known as the ferrous oxalate 
developer. This is prepared by dissolving ferrous oxalate in a saturated solution of 
neutral potassium oxalate, adding the iron salt till no more is taken up. To make up 
the developing solution, equal parts of this solution of ferrous oxalate and of a solution 
of potassium bromide, 20 grains to the ounce, are employed. This mixture is placed 
in a clean developing glass just before development is to take place. The film is first 
softened by flowing over it a mixture of equal parts of alcohol and water, and is then 
well washed. The developer is now poured over the plate, taking care to keep the 
fingers from touching any part of the film. The image will appear gradually, and 
should have fair density when all action is exhausted. 
The intensity can be materially increased by using the ordinary intensifying solu¬ 
tions of pyrogallic acid, citric acid, and silver nitrate. The unreduced silver bromide 
is removed by a saturated solution of sodium thiosulphite in water, from all traces of 
which the film should be thoroughly washed before being allowed to dry.t 
The operation of development should take place in a very subdued red light, that 
recommended for the preparation of the emulsion being the safest.J It is, however, 
somewhat remarkable that when the developing action has once been set up a greater 
quantity of light may be admitted to fall on the plate than before the action com¬ 
mences. The bromide of potassium probably prevents any further action by the 
light, which may account for it. It should be noted that the image may be developed 
by the ordinary alkaline method, though not so satisfactorily, a slight veil being usually 
apparent. 
I may here state that by diminishing the amount of nitric acid to one-fourth the 
amount given in the preparation of the emulsion, it is possible in very cold weather to 
obtain plates which are sensitive to very low radiations, such as the radiations pro¬ 
ceeding from boiling mercury or even boiling water. In summer-time this emulsion, 
as would naturally be expected, produces what are known as “ foggy pictures but it 
can be rendered of use by flooding with hydrochloric acid (see note). In the prepara- 
* It has been found advantageous to coat the plate in red light, and then to wash the plate and 
immerse it in a dilute solution of HC1, and again wash, and finally dry. These last operations can be 
done in dishes in absolute darkness; the hydrochloric acid gets rid of any silver sub-bromide which 
may have been formed by the action of the red light. 
t It aids cleanness of the film if, before drying, a solution of HC1 in water (one to three) is poured over 
it, and afterwards eliminated by washing. This removes any calcium oxalate that may be formed in the 
film. 
+ It can be developed in absolute darkness by using dishes for each operation. 
