That theory and practice are two very different things holds good in
photography especially, and perhaps in no other branch of our art have
so many theoretical formulae been promulgated as in the collotype or
Lichtdruck process. As our readers are aware, we have had an opportunity
of seeing collotype printing in operation in several European
establishments of note, and have, from time to time, published in these
columns our experiences. But requests still come to us so frequently
for information on the process that we have deemed it well to make a
practical summary for the benefit of those who are working--or desire to
work--the method.

The formulae and manipulations here set down are those of Loewy, Albert,
Allgeyer, and Obernetter, four of the best authorities on the subject,
and we can assure our readers there is nothing described but what is
actually practiced.

_Glass Plate for the Printing Block_.--Herr Albert, of Munich, uses
patent plate of nearly half an inch in thickness, as most of his work
is printed upon the Schnell press (machine press). Herr Obernetter, of
Vienna, since he only employs the slower and more careful hand
press, prefers plate glass of ordinary thickness as being handier in
manipulation and better adapted to the common printing-frame.

Herr Loewy, of Vienna, again, uses plate glass a quarter of an inch
thick, as his productions range from the finest to the roughest.

_Preliminary Coating of the Glass Plate_.--Herr Albert's original plan
was to apply a preliminary coating of bichromated gelatine to the thick