THE YOUNG SCIENTIST. 
19 
sists only of a small number of successive 
operations, and the final results are instantly 
obtained; while to make negatives and 
print from negatives demands the full and 
complete mastery of making positives, and 
in case of failure offers the experimentalist 
too wide a range in which to search for the 
causes of his disappointment. The hand 
must be trained to certain manipulations 
and motions, the eye must learn to observe 
and judge, and the necessary discipline of 
the two organs is most conveniently acquired 
in the simple process of making positives. 
The apparatus and materials necessary to 
make positives are: 1. Camera with its lens. 
2. Plateholder. 3. Tripod. 4. Bath. 5. 
Dipper. 6. Several wide- mouthed bottles. 
7. A large and a small funnel. 8. Set of 
scales with weights. 9. A black focusing 
cloth about 1 yard square. 10. Glass plates 
or ferro-plates. 11. One yard of canton 
flannel. 
The chemicals necessary are: 1. Bromo- 
iodized collodion. 2. Pure nitrate of silver. 
3. Iodide of potassium. 4. Protosulphate 
of iron. 5. Acetic acid. 6. Hyposulphite 
of soda, 7. Alcohol. 8. Filter paper. 9. 
Rotten stone. 10. Shellac varnish. 11. 
Bicarbonate of soda. 
The preparatory operations which are re- 
quired before the plate can be exposed, 
must be conducted in the dark room, in 
order to retain the sensitiveness of the 
iodized silver film. The term dark room is 
to a certain extent a misnomer, since the 
dark room is not at all a dark room. At 
the commencement of this article it was 
stated that the actinic force of light is 
found mainly in the blue rays, while the 
yellow rays possess hardly any chemical 
properties. It is therefore only necessary 
to exclude all blue and red rays of light, 
and work with the illuminating power of 
the yellow rays. If we have a dark room 
with several windows, all the windows, with 
the exception of one, should be blocked out 
or darkened with some opaque material, for 
instance, three or four layers of dark-brown 
paper or black oilcloth. The remaining 
window is covered with four folds of yellow 
calico. This yellow curtain need not eover 
the whole window, but only two or threcf 
panes, provided the others are darkened in 
the manner just described. 
In place of this modified daylight, gas or 
lamp light can be used. In this case the 
chimney or globe is surrounded with a yel- 
low, paper screen. Another very practical 
way is to paint three-quarters of the cir- 
cumference of the chimney a dark yellow, 
with an alcohol varnish, to which iodine 
has been added till the desired color is ob- 
tained. By the latter arrangement white 
or yellow light can be thrown at will on the 
plate under opemtion. Have the dark room 
neither too dark nor too highly illuminated. 
In the former case you will not be able to> 
watch the progress of the different oper-^ 
ations; in the latter case all your pictures 
will be covered with an impenetrable veil. 
The dark-room should be provided, near 
the window or lamps, with a table or strong 
shelf, on which to place the necessary chem- 
icals and materials. 
A good supply of water is also indispen- 
sable. For washing smaller plates the 
washing bottle. Fig. 1, answers very well. 
It is constructed in the following manner:. 
Take a wide-mouthed bottle, of a pint, 
capacity, with a tightly-fitting cork. The^ 
Fig. 1. 
Fig. 2. 
cork must be twice perforated to receive two 
glass or tin tubes, a short one, a, and a long 
one, h. As soon as the bottle is turned up- 
side down, as seen in Fig. 2, tube h acts as 
a ventilator, freely admitting air, while out 
of tube a a continuous stream of water will 
flow. 
For larger plates greater quantities of 
water are required. A very simple arrange- 
ment is to take a common waterpail, Fig. 3;. 
