472 PROFESSOR LUDWIG MOSER ON LATENT LIGHT. 
blue, green, yellow and orange glasses. I have had opportuni 
ties of examining a great number of them, and when the opera 
tion was performed in the sun the action was never long in 
making its appearance. The only uncertainty might be with 
regard to the red glasses. I have some of these, in experiment- 
ing with which I thought I observed that they retained the 
iodide of silver in its original colouring, and were not capable of 
commencing the action. On further examination, however, I 
found that this depended on the manner in which I had previ- 
ously made the experiments, inasmuch as | had laid a uniformly 
red coloured glass on the iodized silver plate. In such circum- 
stances it is difficult to observe the slow action of these glasses. 
But if any drawing be made on these glasses, by rendering cer- 
tain parts non-transparent, it will then be seen that they are 
capable of commencing the action just as much as the others, 
although indeed not so rapidly. My friend M. Dulk succeeded 
just as ill in some experiments that he made. He believed that 
chloride of silver does not undergo any blackening under a dark 
red bell jar, even after the lapse of several days; but when I in- 
troduced into this jar an iodized plate, situated behind a screen, 
a perfect picture of the excised part was produced in a few hours. 
All colours then agree with the blue in having the power of 
commencing the action, and of continuing it until blackening 
ensues, although the periods required are very different. Here 
I may be permitted to make a short digression, to obviate a 
deception which might easily mislead. We are accustomed 
to say that blue and violet glasses, which allow but few rays 
of light to pass through them, act powerfully on iodide of 
silver, while yellow and red glasses, which allow the transition 
of a large quantity of light, act only feebly; but I do not 
think that in the present state of the science we may be allowed 
such an expression; by using it we should be placing more 
reliance on the judgement of the eye than it in reality deserves. 
We are well aware that our retina is affected most strongly by 
yellow and orange; so that if by its means we seek to judge of, 
the prismatic spectrum, these colours are always considered as 
the brightest. Ifthe retina were a substance like iodide of silver, 
or if it were freely exposed to the rays of light, the decision 
would certainly be different; under the same circumstances, 
the violet, or Ritter’s dark rays, would appear to it the brightest, 
i.e. those rays which in its actual state produce little or no 
