446 PROFESSOR LUDWIG MOSER ON VISION, 
densing the vapours of mercury. In order to render this more 
plausible, I made a similar experiment with a plate of copper 
during very unfavourable weather. I had not succeeded in 
giving a good polish to the copper, and consequently the image 
produced by the mercurial vapours was slight, although distinctly 
visible. On exposing the plate to vapours of iodine the image 
became very strong, and this method I found very advantageous 
in experiments with copper. 
Finally, I examined the action of light on mirror-glass just in 
the same way; in this case the action was quite as plain as on 
the plate of silver, if the glass was afterwards breathed on; and 
for a long time subsequent I always saw the image produced in 
this manner. If we admit of generalization, the proposition 
would stand as follows :—Light acts on all bodies ; and its influ- 
ence may be tested by all vapours that adhere to the substance or 
act chemically on it. ‘There is no necessity for restricting our- 
selves solely to those vapours which I have examined; without 
doubt hydrofluoric acid might be so used as to exhibit the action 
of light on glass just as well, and at the same time to corrode 
the parts affected. I have not occupied myself with such ex- 
periments, as it appeared of more interest to search for the 
primary laws. 
We now see that the discovery of Daguerre is a special case 
of a very general action; for this instance only teaches us the 
action of light on iodide of silver, as proved by the condensation 
of vapours of mercury. 
Before proceeding further, I thought it necessary to examine 
the action of the different coloured rays on simple substances, at 
least in one instance. Nothing can be said concerning it & prior? ; 
for | have shown above that we cannot ascribe an exclusive che- 
mical activity to the blue and violet rays. My introductory ex-_ 
periments have shown that only the rays of the latter kind exert 
any influence on pure silver, for I obtained very clear images — 
by means of glasses of these colours, while only traces could be 
rendered visible when red glasses were employed, although they — 
transmit more light and heat. However, the weather was ex-_ 
ceedingly unfavourable while I was making these trials, so that” 
I was obliged to give them up for the time. 
I will now describe the manner in which the different vapours 
are condensed by the plates. My first experiments with the 
breath tended ‘to discover whether the untouched parts, the 
