AND ON THE ACTION OF LIGHT ON BODIES. 441 
those here mentioned, and we must therefore seek elsewhere for 
the means by which the sensibility of the retina is modified, i. ¢. 
increased or diminished, and by which the sensitiveness which 
has been reduced may be brought back to its normal state. 
In the present research I was partly led by the idea that 
pressure might be the means in question; and I was thereby 
induced to try whether something similar might not be the case 
with other bodies sensitive to the action of light. I have suc- 
ceeded in discovering a series of facts, and some general laws, 
which it appears to me may be classed among the most extraordi- 
nary phenomena in this department, and which promise to ex- 
tend our views. I will describe these facts in the same order as 
they presented themselves to me, which plan may perhaps give 
the best general view of the subject. 
It is a well-known fact, that if we write with certain substances 
on a well-polished glass plate, and then clean it, the characters 
always make their appearance again by means of the aqueous 
vapours, if we breathe over it. The experiment has also been 
made by placing a coin on a glass plate, breathing on it, and 
then removing the coin ; if the plate was afterwards breathed on 
the figure of the coin appeared, although certainly only in 
outline. 
This is all that I have found of this particular kind. But the 
experiment has been made much toe confined; we may write on 
the glass plate with any substance whatever, the after-phzno- 
mena always remain the same. We may first breathe uni- 
formly over the whole plate, and then write on it either with 
blotting-paper, a brush, or anything else; the characters will 
become visible whenever the plate is breathed on, and this phe- 
nomenon lasts for some time. Not only is glass applicable to 
this purpose, but every other polished body exhibits the same 
appearance : I have tried it with metals, resins, wood, pasteboard, 
leather, &c. Even fluids may be used; if we take a very clean 
and still surface of mercury, hold over it a body, and breathe on 
the other parts, or what is better, breathe on the whole surface 
first, and then remove the moisture by any gentle means from 
particular parts, they will again become visible when breathed 
on, even after several days, if the mercury remain undisturbed. 
Moreover, absolute contact with the extraneous body is not ne- 
cessary, whether it be before the breathing, or only to remove 
the precipitated moisture from particular parts. If we hold over 
VOL. III, PART XI. 2G 
