are capable of undergoing in Darkness. 275 
the most unwearying perseverance in experimental observa- 
tion, and by a mind capable of drawing the most logical de- 
ductions therefrom. I state them now as they have occurred, 
for the purpose of showing how perfectly independent these 
effects appear to be of the chemical rays of light. I may 
mention another very curious result in connexion with the 
above. ‘Two copper-plate engravings were placed upon 
highly polished amalgamated plates of copper; one of them 
was covered with common window-glass, and the other with 
a deep red glass stained with the oxide of gold. They were 
exposed to daylight for four hours, during which there were 
but faint gleams of sunshine. On subjecting the plates to the 
vapour of mercury, a very capital copy of the print was found 
to have been made under the influence of the red glass, but 
no trace of an impression under the other. It will be seen, 
on referring to Dr. Draper’s paper, that these results corre- 
spond mainly with some obtained by him; but they will not 
admit of the explanation he has given of the radiation of 
tithonicity, unless he can in the first place prove the constant 
presence of the agent he has so named in all bodies. I know 
he attempts to do this, but I am by no means satisfied that 
either Moser or Draper have as yet proved either the ex- 
istence of * invisible light,” or ‘dark tithonic rays.” 
With the view of testing Dr. Draper’s results, I carefully 
iodized two silver plates and exposed them to light. I then 
placed them so that half of one plate was covered by half of 
the other, and allowed them to remain in the dark ,1,th of an 
inch apart for four hours. On mercurialization I could not 
detect the slightest difference between the covered and the 
uncovered portions of either of the plates. 
Another silver plate was iodized and exposed to light.. It 
was then placed in the dark with a sensitive plate which had 
been carefully kept from the light ;4,th of an inch above it, 
and a small engraving placed between them. They were al- 
lowed to remain thus for six hours. When exposed to the 
vapour of mercury, the plate which had been subjected to the 
light whitened all over, and the space occupied by the en- 
graving was distinctly marked by lines of vapour thicker than 
the other parts. The plate which had been preserved in the 
dark was scarcely at all influenced by the vapour, except on 
those parts which had been touched by the supports of card- 
board on which it rested. ‘These were so arranged that no 
radiation could have influenced those parts of the plates, 
An iodized silver plate was placed in the dark with a little fine 
string coiled over parts of it, and a polished silver plate sup- 
ported jth of an inch above it. After four hours both plates 
T2 
