426 PROFESSOR LUDWIG MOSER ON VISION, 
believes that the above-mentioned fact proves the existence of 
two distinct kinds of chemical rays, and that it is therefore neces- 
sary to distinguish between, 
1. Exciting rays (rayons excitateurs). 
2, Continuing rays (rayons continuateurs). 
The blue and violet rays belong to the first class, and the red 
and yellow to the second. 
These experiments I have on the whole been able to confirm. 
A plate which required eight minutes’ exposure in the camera 
obscura, in order that it should give the proper image with mer- 
curial vapours, was taken out at the end of one or half a minute, 
and laid under a red glass in the sun for two minutes. On being 
exposed to the vapours it gave a perfectly correct image. I men- 
tion the spaces of time merely for the sake of examples, inasmuch 
as no general rules can be given on this point; and I will only 
add, that when I interrupted the action of the camera obscura 
too soon I obtained no correct image, although my red glasses 
allowed very few blue or violet rays to pass through them. If 
the plate lay longer under the red glass, a negative image was 
produced, and this circumstance can often be advantageously 
employed to determine whether any action has taken place on 
the iodide of silver without having recourse to the vapours of 
mercury. ‘ 
Very different, however, are the effects of yellow glasses. I 
possess some which exhibit a pure yellow, and others which have 
a reddish tinge. The latter act like the red glasses, only they 
allow a large quantity of white light to pass through, which 
blackens the iodide. Ifa prepared plate be covered with a yel- 
low glass, after it has been in the camera obscura for about the 
proper time, and be then placed in the sun, a curious pheenome- 
non is observed. While the plate exhibits at first no image 
whatever, a negative one is now rapidly produced; this vanishes, 
and in about 10 or 15 minutes a positive image is formed. 
Under a yellow glass these positive images always have a blackish 
covering, but with this exception are perfectly clear and well- 
defined. I could never obtain these positive images under red 
glasses, even by the most continued action of light; but they 
are easily produced by green glasses, although not so quickly as 
by yellow ones. - 
If we attentively consider all these facts, we shall arrive at a 
conclusion concerning the action of the several colours of light, 
