428 PROFESSOR LUDWIG MOSER ON VISION, 
much as these rays only begin to act when all the others can no 
longer exert any influence. We may therefore use common white, 
undecomposed light, and we shall thus be led to the following 
striking experiments, which are easy enough of confirmation, 
An iodized plate was laid in the sun until it was blackened, 
which took place very quickly. Half of it was now protected 
from the sun; after a few minutes it was evident that the un- 
covered part was strikingly lighter than the covered portion. 
This decolorizing process lasts some time, and finally, the plate 
which was at first black assumes a greenish-yellow tinge when 
viewed by reflected light. This action depends upon the sun’s 
green and yellow rays, and it is clear that sunlight acting unin- 
terruptedly on yellow iodide of silver, first makes it dark and 
then restores its colour. 
An iodized plate of silver was exposed to the sun until the 
blackening no longer increased, and was then put into a camera 
obscura directed towards some houses; after a lapse of twenty- 
four hours it was removed, and exhibited, as might be expected, 
a correct positive image. The light parts of the object had in 
the image a steel-gray colour, the dark parts were black. 
The following experiment is very striking, although easily to 
be explained from the above-mentioned action of the green and 
yellow rays. An iodized plate is put into the camera obscura 
on a day when the sun shines, and allowed to remain in it for 
half or a whole hour, or even longer, so that a strongly marked 
negative image is produced. The plate is now to be laid in the 
sunshine, and a very beautiful phanomenon will be observed. 
After a few minutes the negative image vanishes and an equally 
strong positive one is produced, in which the bright parts are 
blueish-green and the dark reddish-brown. From all these ex- 
periments it will be seen, that the question as to which colour of 
light operates chemically, cannot be answered in the abstract, as 
has been fully shown by the yellow and black iodide of silver. 
As yet our knowledge of bodies in this respect is very deficient, 
and for the present we must leave it undetermined whether the 
law discovered as applicable to the iodide of silver is general, 
with slight variations, however, as regards this or that colour. 
On this point I have ouly examined the alcoholic tincture of 
guaiacum, which certainly offers very analogous appearances. 
Papers stained with this solution have at first a reddish colour, 
which becomes tea-green in daylight. The rays which act first 
