Just a word as to the preparation of the glasses before coating. It is
very generally considered that it is better the glasses receive either a
substratum of albumen or very weak gelatine. I use the latter on account
of the great ease of its preparation. After your glasses are well
cleaned, place them in, and rub them with a weak solution of hydrochloric
acid of the strength of 2 ounces acid to 18 ounces water.

Prepare a solution of gelatine 1 grain to the ounce of water, rinse the
plate after removal from the acid mixtures, and coat twice with the above
gelatine substratum; the first coating is to remove the surplus water,
and should be rejected. Rear the plates up to drain, and dry in a plate
rack or against a wall, and be careful to prevent any dust adhering to
the surface while wet.

Having now described the plates I intend to use, let us next consider
what a transparency is, that we may understand the nature of the work we
are undertaking. You are all aware that if we take a negative, and in
contact with it place a sheet of sensitized paper, we obtain a positive
picture. Substitute for the paper a sensitive glass plate, and we obtain
also a positive picture, but, unlike the paper print, the collodion or
other plate will require to be developed to bring the image into view.
Now this is what is termed making a transparency by contact. It often
happens, however, that a lantern slide 3¼ by 3¼ has to embrace the whole
of a picture contained in a much larger negative, so that recourse must
be had to the camera, and the picture reduced with the aid of a short
focus lens to within the lantern size; this is what is called making a
transparency by reduction in the camera. Both cases are the same,
however, so far as the process being simply one of printing.