The last material we require is the gelatine, one of the most important,
and at the same time the most difficult substance to obtain of good
quality. I have various samples here--notably Nelson's No. 1 and "X
opaque;" Coignet's gold medal; Heinrich's; the Autotype Company's; and
Russian isinglass.

The only method I know of securing a uniform quality of gelatine is to
purchase several small samples, make a trial emulsion with each, and buy
a stock of the sample which gives the best results. To those who do not
care to go to this trouble, equal quantities of Nelson's No. 1 and
X opaque, as recommended by Captain Abney, can be employed. Having
selected the gelatine, 11/4 ounces should be allowed to soak in water, and
then melted, when it will be found to have a bulk of about 6 ounces.

In order to prepare our emulsion, I take equal bulks of the silver
nitrate and potassium bromide solutions in beakers, and place them in
the water bath to get hot. I also take an equal bulk of hot water in a
large beaker, and add to it one-half an ounce of the gelatine solution
to every 12 ounces of water. Having raised all these to about 180 deg. F., I
add (as you observe) to the large beaker containing the dilute gelatine
a little of the bromide, then, through a funnel having a fine orifice,
a little of the silver, swirling the liquid round during the operation;
then again some bromide and silver, and so on until all is added.

When this is completed, a little of the emulsion is poured on a glass
plate, and examined by transmitted light; if the mixing be efficient,
the light will appear--as it does here--of an orange or orange red
color.