A short chapter may be devoted to the various suggested processes for producing Collotype in natural colours, with which are associated the names of Vidal, Albert, and Obernetter. To give a complete _resume_ is altogether impossible, as up to the present time the inventors have only published their method of procedure with very considerable reservations, and notwithstanding the fact that a great deal has been written and published in current literature on the subject, little is actually known. The methods of Vidal and Albert are essentially alike, both being founded upon an opti-chemical basis. By careful registration and printing from three Collotype plates, representing fragments of the same subject upon one sheet of paper, and making use of inks corresponding with the three primary colours, a total effect should be produced which ought, theoretically, to resemble the coloured original. In preparing the three negatives for reproducing the three colour plates a special mode of procedure must be adopted. Each of the three negatives must give a resulting Collotype plate capable of retaining varying amounts of ink when rolled up, according to the predomination in the original of one or the other of the primary colours—red, blue, or yellow.[M] This is effected in the production of the negative for the red plate, by making the exposure through a green glass screen, while the negative intended to produce the plate from which the yellow is to be printed is exposed through a screen of violet. [M] Pictures produced in the three primary colours are extremely crude. Dr. Vogel has proposed a large number of negatives, using for their production sensitisers corresponding with every region of the spectrum. For example—Napthol blue for red, cyanin for orange, rozin for yellow, and fluorescein for bluish green, the ordinary sensitiveness of the plate being sufficient for blue and violet, the latter, however, being exposed through a yellow screen. The images thus obtained are printed from lithographic stones or Collotype plates, each of which is printed in a colour complimentary to that part of the spectrum to which the particular plate was sensitive. The greater the number of separate images produced in this way the more complete will be the reproduction of the various shades in the original, and the more pleasing the resulting print. The green screen allowing to pass so few chemically active rays the exposure has to be prolonged to such an extent that gelatine bromide dry plates stained with eosine must of necessity be used.