460 PROFESSOR LUDWIG MOSER ON VISION. 
fore the experiments are made. After the plate of agate had — 
been many days removed from light, I placed it half covered in 
the sun’s rays. On being afterwards depicted on a plate of 
silver, no perceptible difference could be observed between the 
covered and the uncovered half. 
If we admit this self-luminosity of bodies, and assume, more- 
over, that which I think I have been able to prove elsewhere, 
viz. that those rays which proceed from the surface in a slanting 
direction exert a much smaller degree of influence than those 
which take a perpendicular course, then there is no difficulty 
in explaining the clearness of the images even when formed at 
a considerable distance. We also easily perceive why heating 
the depicting body may sometimes be advantageous ; because, 
as heat employed in a proper degree produces incandescence, 
so it is natural to infer that the inferior degree will produce a 
self-lurminosity, and therefore increase the influence in question. 
The power of self-luminosity seems to be present in bodies in 
very different degrees, and the polish seems to exert a percepti- 
ble influence on it, although comparative experiments in this 
respect will be very difficult. 
As yet I] have depicted the following substances on pure silver: 
Pure silver, iodized silver, brass, iron, steel (particularly the stamp 
of a coin), violet and red glass, black polished horn, white paper 
with characters inscribed, gypsum, mica, agate and cork, and as 
yet I have not met with any body with which the experiment 
did not succeed. As the list of bodies is varied enough, we may 
assume that the property of self-iuminosity is inherent in all 
matter. Under certain circumstances the force must be re- 
garded as very considerable, for I obtained the image of several 
bodies on a plate of silver in the course of ten minutes, which 
proves the existence of a very powerful action. As the bodies 
depicted were partly dark ones, we arrive at the conclusion, that, 
even where for the retina only darkness is present, there is still 
a considerable evolution of light, which proves its presence by 
acting on suitable substances. It is true that in so short a time 
as that above named images can only be obtained by the con- 
tinuous action of mercurial vapours of a low tension; because, 
as I have shown, these vapours, as indeed all, are capable of con- 
tinuing the action commenced by light. In the preceding ex- 
periments, however, no vapours are necessary, and if an iodized 
plate be brought near any body in the dark, and sufficient time 
