AND ON THE ACTION OF LIGHT ON BODIES. 459 
ADDENDUM*, 
In my treatise on Vision, &c. I have shown that contact pro- 
duces the same effect as light, and that in this manner we may 
obtain a very accurate image of the body in contact. The accu- 
racy with which engraved figures, for instance, are depicted in 
successful experiments, rendered it very improbable that a per- 
fect material contact is necessary, and I have since convinced 
myself that such contact did not really take place in most ex- 
periments. Neither the bodies themselves, nor the silver plates 
on which they were depicted, were so even as to allow of the 
possibility of complete contact ; consequently an action seemed 
here to have been exerted at a distance. A plate of agate with 
several engraved figures was covered with thin strips of mica, and 
upon these the silver plate was laid, so that the space between 
the two surfaces amounted to one-fifth of a line, and admitted 
of seeing through; when, after the lapse of several hours, the 
plate was introduced into the mercurial vapours, a perfect image 
of the engraved figures was produced. I have examined other 
bodies placed at a greater but not measured distance, and always 
found them depicted, and have thereby discovered the curious 
fact, that when two bodies are sufficiently approximated they re- 
ciprocally depict each other. It is necessary to remark, that all 
extraneous light was carefully excluded in these experiments; 
that I made these experiments during the night, and without 
candlelight ; that the bodies were placed in a dark box in a dark 
chamber; and, which is the principal point, that all other light 
falling upon these approximated bodies would render the forma- 
tion of images quite inexplicable. One kind of experiment is 
to keep the bodies apart from each other, the other is to bring 
them into close contact, as for instance a plate of silver lying 
upon one of agate, or anything else; how can any extraneous 
light act in this case? Notwithstanding, I do not believe that 
the existence of a new power belonging to bodies has been hereby 
proved ; I am much rather of opinion that these experiments lead 
us to another result, viz. that every body must be considered as 
self-luminous. "This property, as far as my experiments go, is 
not first produced by extraneous light, as is the case with phos- 
phorescence ; it makes no difference whether the bodies have 
been kept in the dark for a long time or exposed to the sun be- 
* From a letter of the Author, dated 2nd of June. 
2H 2 
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