1908.] Alled Bodies on a Photographic Plate in the Dark. 379 
found that its activity has been greatly increased. Fig. 3 (Plate 11) shows the 
picture given by the resin in its ordinary state, and after exposure to the 
arc light for half an hour. In one experiment a slab of resin was exposed 
to a bright July sun for 5 seconds, and this caused no increase of activity ; 
but exposed for 15 seconds and a slight increase occurred, and after an 
exposure of 30 seconds the activity of the slab had greatly increased ; but 
on a still longer exposure no further increase took place, so that in about 
30 seconds the resin was charged to its maximum amount. Another 
experiment, when the exposures were for 1 minute, 5 minutes, and 
15 minutes; the 1 minute and the 15 minutes gave similar results. If 
the arc light be used in place of sunlight the same kind of action occurs, 
only more slowly. In one experiment the exposures were for 5 seconds, 
15 seconds, and 30 seconds, and no marked increase of activity took place, 
but after 60 seconds’ exposure a great increase was evident. With another 
sample of resin exposed to an are light it was found that it required 
5 minutes’ exposure to obtain its greatest amount of activity. 
If amber resin, in place of ordinary resin, be used, it requires a longer 
exposure to light to charge it to its greatest amount. Resin is, however, a 
body which varies so much in composition and constitution that exact 
measurements cannot be relied on for different specimens, and the above 
experiments are only intended to show the nature of the action which occurs. 
The effect of heat on resin in its ordinary state has already been described ; 1f, 
now, a resin slab, charged to its maximum by exposure to light, be heated 
to 55° for only one and a-half hours, all this extra activity is destroyed and it 
returns to its original state of activity. 
This increased activity induced by light acts generally in the same way as 
the original activity of the resin: it is destroyed by sulphur dioxide and does 
not pass through glass, mica, etc. In all cases of stimulating resin by the 
action of light a short interval occurs after the application of the light and 
before the increase of activity begins; when once begun, the increase takes 
place rapidly and it soon becomes charged to its maximum, so that longer 
exposure produces no further increase of its activity. 
In order to ascertain which rays of the spectrum were most active in 
producing this change, a spectrum obtained from an arc lamp, with a quartz 
‘prism and lens, was allowed to fall upon a slab of resin for one and. 
a-half hours, and this gave, after contact with a photo plate for two hours at 
40° C., evidence of action having taken place where the blue rays had fallen 
on the resin and not elsewhere. On placing slabs of resin in double bell jars 
with different coloured liquids, the blue, a solution of ammonia sulphate of 
copper, and the red, potassium bichromate, it was found that, even after an 
