The essential new feature of "Unitol" is that, unlike most fine-grain developers, it may be diluted within wide limits so that it becomes possible to use it with the utmost economy – and simplicity – by following the slogan "One ounce; one film; use once; then discard". It is reckoned that one ounce of the concentrated solution is necessary for the proper development of one film, and that it is by then exhausted for practical purposes…Quite apart from this convenience, however, "Unitol" is a fine-grain developer of the highest grade…A most convenient calculator and film group chart has been prepared for use with "Unitol". Sold at the nominal price of 4d, it enables the time of development for any dilution, at any temperature, of any film to be immediately calculated by the rotation of a disc.
Meritol was formed from paraphenylenediamine and pyrocatchin (pyrocatechol, catechol), each a developing agent. The additive product is a new compound with different properties from each of its constituents. Claims that the two components can be used instead of Meritol are not, therefore, accurate. Research on Meritol continued. I even found a paper in a chemical journal from 1989 that shows the distinct structure of Meritol.
Finding the Unitol Calculator (which would now of course be launched as an app) was a reminder of that 1950s mantra: Ilford Film; Kodak Paper; Johnsons Developer. It has to be said though that D76/ID11 caused wavering with the last part.
|Johnsons of Hendon advertisement in BJPA 1951|
|Centre was pushed out to form the|
central disc of the calculator