PROFESSOR LUDWIG MOSER ON LATENT LIGiT. 473 
effect on it, would then exert the strongest action. It is just 
the same with the retina and the different compounds of silver 
as with the other bodies which have as yet been exposed to the 
action of light; it has always been found that the rays of a par- 
ticular period of oscillation are the most energetic in their action. 
This appears to me to prove both that these bodies and the re- 
tina are by no means adapted to the determination of the relative 
intensities of the different coloured lights, and that therefore no 
very great degree of faith can be reposed upon the decision of the 
latter, viz. that a violet glass transmits few, and a yellow many 
rays. Apart from this, we find that rays of different degrees of 
refrangibility do not agree in regard to the blackening of the 
iodide of silver; certainly in respect to the time necessary for 
the operation, but in nothing else. And now we see that this 
effect is connected with the duration of oscillation; so that we 
May say, the greater the duration for one group of rays the 
longer space of time will they need to commence action on the 
iodide of silver, and to continue it until blackening ensues. 
I now return to the peculiar power which red rays are said to 
have of continuing an action which has been already begun. In 
my treatise on Vision I have raised some doubts with regard to 
the peculiar action of this class of rays, and I have mentioned 
the principal argument by which such an idea would be sub- 
verted; but among so many others I did not direct more par- 
ticular attention to the importance of that fact. It is true, that 
if the operation, as for instance the production of a Daguerre’s 
picture, be performed in the common rays of sun- or daylight, 
then the red rays have a powerful continuing action, for it was 
in this case commenced by the blue, violet, and Ritter’s dark 
rays. But now [ have discovered and described another class 
of rays of light, those emitted by every body without exception, 
_ because it is self-luminous,—rays whose presence is evidenced by 
the fact of two sufficiently approximated bodies impressing their 
images on each other, although everything that the retina could 
denominate light has been excluded. I call them the invisible 
rays of light, to distinguish them from Ritter’s dark rays at the 
violet end of the spectrum ; I might also call them the most re- 
frangible rays, for it appears that their refrangibility is greater 
than that of the other rays of the spectrum. As I shall show, 
they are not present in day- and sunlight, and must not there- 
fore he confounded with the above-mentioned dark rays. When 
VOL, III. PART XI. 21 
