THE insensitivity of the photographic plate in recording positive rays when compared with its sensitivity to light has long been observed, and has been accounted for by the fact that the action of positive rays is purely a surface effect. There has, therefore, always been the hope that considerable improvement could be made in this direction by increasing the concentration of the bromide particles on the surface of the gelatine. This hope has now been realised to some extent by the use of a method which, I understand, has been devised for the production of Schumann plates. It consists essentially in dissolving off more or less of the gelatine by means of acid. I have not yet succeeded in obtaining certain or uniform effects, but in the most favourable cases the sensitivity of the “Half Tone” plates used in the mass-spectrograph has been increased ten to twenty times without seriously altering their other valuable properties.
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