1904.] on a Photographic Plate in the Dark. 133 



Some of the resin in immediate contact with the knot is in some cases 

 but little active. The marked difference in properties of resins from 

 different sources is described, and it is shown how difficult it is to 

 remove it, so that the wood shall be no longer active. Boards that 

 have been exposed to the air for a long time, an oak box a hundred or 

 more years old, rotten wood from the stump of a tree, and even bog 

 wood have all been found to be still active. 



In addition to woods many different resins and allied bodies can, 

 when used alone, be proved to be very active, some naturally much 

 more so than others. Ordinary resins, Burgundy pitch, gum mastic, 

 .are very active, asphaltum, dragons blood much less so, but true gums 

 such as gum Senegal and gum arabic are entirely without action on 

 a photographic plate. 



In certain cases the picture obtained on the plate does not resemble 

 the markings which are visible on the wood. With some woods this 

 more commonly occurs than with others. That this picture is 

 persistent in the wood is shown by fresh sections giving the same 

 result. The true bark of a wood is apparently quite without action 

 on a photographic plate, so is the internal pith of a plant. 



There is another and a very interesting action, which occurs 

 with wood, it is the great increase of activity which it exerts on a 

 photographic plate after it has been exposed to a strong light. 

 For instance, if a piece of deal be half covered by black paper or 

 tin foil and be exposed! for 5 10 minutes to bright sunlight, and 

 then put up in the usual way with a photographic plate, it will give 

 a dark picture where the light has fallen on the wood and only a 

 very faint picture of the part which has been covered. This is 

 shown in Plate 7, fig. 1. Even comparatively inactive woods such as 

 elm and ivy after a short exposure to bright light give good and dark 

 pictures. The action is not an indiscriminate darkening over the 

 whole wood section, but an intensifying of the parts already active. 

 This increase of activity by the action of light appears to occur with 

 all woods. Artificial light, such as that from the electric arc, or from 

 burning magnesium ribbon, act in the same way, so does even a faint 

 light. A piece of wood put at a window for some hours will give a 

 darker picture than a similar piece left in the middle of the room. This 

 increase of power of a wood to produce a picture does not rapidly pass 

 away. After 24 hours the action is visibly less, and decreases more 

 rapidly at first than after some days, but it will be a fortnight or may 

 be a month before the wood resumes its former condition. This 

 action, like the former one, is entirely stopped by interposing the 

 thinnest piece of glass or mica between the photographic plate and 

 the active body. An inactive card painted with an alcoholic solution 

 of resin, acts in the same way, and turpentine which has been exposed 

 to a bright light acts more strongly on a photographic plate than it 



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