496 



Major-General J. Waterhouse. The Sensitiveness 



the cut-out paper mask and mica screen, in which were cut-out initials, 

 just as if it were going to be exposed in the sun, but it was gently 

 warmed for about five minutes over a spirit lamp, and then developed 

 with mercury. The cut-out initials and edges of the mica came out 

 distinctly in dark lines, just as they did in the pressure experiment 

 but there was also a clear image of the black paper mask, which 

 developed lighter than the ground, by the deposition of mercury, or 

 the opposite of the ordinary action of light. 



This is a very interesting observation, but recent repetitions of it 

 with silvered glass plates and clean silver foil have quite failed to give 

 such a distinct image of the black paper, though traces of it have been 

 visible, and the edges of the mica and of the cut-out initials were always 

 clearly impressed. In one case, when the silver foil was well heated 

 to redness on both sides before being placed in the printing frame and 

 the subsequent heating, no image of the initials was obtained, and only 

 part of one edge of the mica screen with a faint trace of one corner of 

 the paper mask where there was extra pressure. From this it would 

 seem that heat does not play any active part in the production of the 

 images, though the higher temperature of the summer sunshine, as 

 well as its greater actinic power, may accelerate their formation by 

 light. This is to a certain extent proved by the fact that the most 

 perfect printed-out image obtained on a pure silver plate was exposed 

 for three days at the end of September, when the thermometer 

 exposed in the sun at the same time did not rise above 64° F., so that 

 there could have been no question of heat producing the effect, as there 

 might have been under the hot, clear sunshine of August. 



Protection from Air. — In most of the experiments the plates were 

 protected by glass during exposure, so that the outer air had no 

 direct access to them. AVhen plates were exposed under mica screens 

 and without the protecting glass, the outside unprotected surface 

 became distinctly yellow and tarnished during long exposures. 



Under Surface of Silvered Glass Plate not Sensitive. — In order, however, 

 to ascertain the effect of cutting off all atmospheric action on the 

 exposed side, a silvered glass plate was exposed from the back or glass 

 side under a cut-out mask made of thin aluminium sheet, and exposed 

 for four days in October, two days being sunny and two cloudy. 

 There was no msible image on either side of the plate after exposure, 

 but breathing showed an image on both sides. The plate was then 

 developed with acid iron and silver, and showed the image not very 

 distinctly, but as development was prolonged traces of it appeared at 

 the back of the silver film quite clear of deposit, so that apparently 

 the developer had worked through the protected parts of the film, 

 while the exposrd part had taken the deposit of silver. When the plate 

 was dry there was, curiously enough, no trace of the image on either 

 side of the plate. 



