THE 
LONDON, EDINBURGH anp DUBLIN 
PHILOSOPHICAL MAGAZINE 
AND 
JOURNAL OF SCIENCE. 
SUPPLEMENT to VOL. XXII. THIRD SERIES. 
LXXXIV. On certain improvements on Photographic Pro- 
cesses described in a former Communication, and on the Para- 
thermic Rays of the Solar Spectrum. By Sir J. F. W. Her- 
SCHEL, Bart., K.H., F.R.S., &c.; in a Letter addressed 
to S. Hunter Christie, Hsy., Sec. R.S. Communicated by 
S. Hunter Christie*. 
Dear Sir, 
231. [ BEG leave herewith to submit for your inspection 
and that of the Royal Society, a series of photogra- 
phic impressions illustrative of the chrysotype, cyanotype, and 
other processes, an account of which is given in the Post- 
script to my last paper on that subject, which has, by per- 
mission of the President and Council, been appended to the 
original in its printed form subsequently to the termination of 
the Session. In the interval which has since elapsed, besides 
the discovery of other photographic novelties (which may form 
the subject of future communications), I have been enabled 
materially to improve some of the processes there described ; 
and these improvements, with a few remarks on some other 
points treated of in that paper, in relation to the processes in 
which the thermic rays are concerned, are now subjoined. 
232. ‘The positive cyanotype process described in Arts. 219, 
220 of my papers, though beautiful in its effect (especially 
during the first few minutes of the appearance of the picture), 
is very precarious in its ultimate success, owing to causes 
there detailed. The remedies proposed are also only occa-~ 
sionally and partially successful, and in consequence this pro- 
cess, though exceedingly easy in its manipulations, could not 
be recommended as practically useful. After trying a vast va- 
riety of means to overcome these obstacles to its success, I 
* From the Philosophical Transactions for 1843, Part I., having been 
received and read by the Royal Society, November 17, 1842. This paper 
is in continuation of that concluded at p. 252 of the present volume. 
Phil, Mag. S. 3. No. 148. Suppl. Vol. 22. 2M 
7. 
