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Desensitizing Plates 297
to the plate. The intermediate compounds, which precede the formation of phenosafranin, phenyl blue, Bindschedler green, toluylen blue, phenazin, toluylen red and neutral violet were examined, and were found without action, except toluylen red, which acts very much like phenosafranin ; paraphenylendiamin was also found to have slight desensitizing action. It appears that the presence of the phenazin group, and the substitution of amino groups in the benzol nucleus are essential.
Of the various dyes, it was found that the following acted the same as phenosafranin : dimethyl-phenosaf ranin, tetra-ethyl-phenosaf ranin (amethyst violet), dimethyl-benzo-xylylsafranin (girofle), tolusafranin (ordinary safranin), methyl-tolusaf ranin (saf ranin MN) and phenonaphtho and creso-saf ranin ; whilst ethyl-dimethyl-ethosaf ranin (fast neutral violet D), tolu-naphtho-etho-saf ranin (indulin scarlet), tetra-methylphenosafranin were slightly inferior. Safranone, amidosafranon, homosaf ranin (azo-carmin X) were much inferior. Dimethyl-amino-phenyldimethyl-benzo-saf ranin (indazin M), a diazo derivative of phenosafranin, and the a-naphthol derivative (indoine blue R) were also much inferior and attacked the image. Acetyl-phenosafranin and safranol were without action. Of the above dyes, comparable with phenosafranin, the creso-safranin has the advantage of being more readily washed out of the gelatin. The indulins, fast blue R, soluble indulin blue B, paraphenylen blue R, metaphenylen blue 2B, azin green and azin green S, Bale blue R, milling blue and Coupier's blue had no action, neither did Lauth's violet, Nile blue 2B, nor resorufin, the first being a thiazin and the others oxazin or oxazon dyes of constitution approximating the safranins. Picric acid, aurantia, Indian yellow and chrysoidin were found to be desensitizers.
There is apparently no well defined relation between the constitution of the dye and the desensitizing properties. A large number of nitrogenous substances, especially the alkaloids, were also tested, and apomorphin hydrochloride was alone found to be active ; its solution after turning blue by exposure to the air, seems to be more active than that freshly prepared.
To test the action on panchromatic plates, a particular kind, sensitive to wave-length 7000, was exposed in the spectrograph, then immersed in the dark for 1 minute in the following solutions : phenosafranin and other safranins and chrysoidin 1 : 2000; aurantia, toluylen red and apomorphin, 1 : 1000; picric acid and diaminophenol, 1 : 100; neutral potassium chromate and Indian yellow, 2 : 100. Afterwards the plates were developed for \l/2 minutes in the dark with normal amidol developer, and then for 2 minutes at a distance of from 18 to 48 inches from a 16 cp. incandescent bulb, screened with tartrazin paper, and examined four times, for 3 seconds each time, by transmitted light. Under these conditions, the plates treated with the various safranins, toluylen red and aurantia gave only very slight fog; whilst all others were strongly fogged.