The Rev. M. O’Brien’s Reply to Prof. Kelland. 21 
the picture again faded, by very slow degrees, and on January 
2, 1842, was found quite obliterated. Being then again sub- 
jected to the acid vapour, the colour was reproduced. How 
often this alternation might have gone on I cannot say, the 
specimen having been mislaid or destroyed. But a portion 
of such paper photographically impressed with a stamped 
pattern, accompanies this communication for the satisfaction 
of any member who may wish to try the experiment. ‘The 
extreme slowness of the action precludes any prismatic ana- 
lysis of the process, and it cannot be too often repeated that 
the use of coloured glasses in such inquiries serves only to mis- 
lead. Of dormant photographic impressions generally, whether 
slowly developing themselves by lapse of time, or at once re- 
vivable by stimuli, as well as of the spontaneous fading and 
disappearance of such impressions, I shall have more to say 
hereafter, having encountered several very curious cases of 
the kind in studying the habitudes of gold, platina, &c. I 
would here only observe, that a consideration of many such 
phznomena has led me to regard it as not impossible that the 
retina itself may be photographically impressible by strong 
lights, and that some at least of the phenomena of visual spec- 
tra and secondary colours may arise from the sensorial per- 
ception of actual changes in progress in the physical state of 
that organ itself, subsequent to the cessation of the direct sti- 
mulant. 
[To be continued. ] 
III. A Reply to Professor Kelland’s Observations in the Phi- 
losophical Magazine for November 1842. By the Rev. 
M. O’Brien *. 
AS my final reply to Professor Kelland, I beg to state very 
briefly the following facts :— 
Ist. Professor Kelland has given no answer to the questions 
I asked him, viz. “ Why did he bring forward an expression 
for v? from his ‘ Theory of Heat’ as equivalent to mine, when 
it most clearly does not account for dispersion independently 
of the hypothesis of finite intervals?” And again, “ Why did 
he not quote the words which follow this expression in his 
‘ Theory of Heat,’ containing his own admission that it was 
too uncertain to be made use of?” ‘These questions contain 
my chief “ charges” against Professor Kelland; and since he 
has not answered them, I conclude that he cannot deny his 
attempt to create an impression that he had, in his * Theory 
of Heat,’ anticipated my explanation of dispersion. 
* Communicated by the Author. 
