PHILOSOPHICAL TRANSACTIONS. 
I. On certain improvements on Photographic Processes described in a former Commu- 
nication, and on the Parathermic Rays of the Solar Spectrum. By Sir J. F. W. 
Herschel, Bart., K.H . , F.R.S.,8>c. ; in a Letter addressed to S. Hunter Christie, 
Esq., Sec. R.S. Communicated by S. Hunter Christie. 
Received November 17, — Read November 17, 1842. 
Dear Sir, 
231. I BEG leave herewith to submit for your inspection and that of the Royal 
Society, a series of photographic impressions illustrative of the chrysotype, cyano- 
type, and other processes, an account of which is given in the Postscript to my last 
paper on that subject, which has, by permission 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 appear- 
ance of the picture), is very precarious in its ultimate success, owing to causes there 
detailed. The remedies proposed are also only occasionally and partially successful, 
and in consequence this process, though exceedingly easy in its manipulations, could 
not be recommended as practically useful. After trying a vast variety of means to 
overcome these obstacles to its success, I have succeeded at last, by the simple addition 
of corrosive sublimate to the ammonio-citrate of iron with which the paper is pre- 
pared. The improved process, therefore, may be thus stated. Mix together equal 
measures of a saturated cold solution of corrosive sublimate, and a solution of am- 
MDCCCXLin. 
B 
