52 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
the ocular evidence of their effects thus afforded must at once set the question at rest. 
But the extent to which the thermic rays thus traced, overlap the luminous and even 
the chemical or oxidizing rays, is, I must confess, to myself quite unexpected. 
128. If we take in the calorific effect at the least refrangible or negative end, and 
the chemical at the positive end of the spectrum, the one as traced in the above ex- 
periments, the other as in the photographic spectrum of bromuret of silver (Art. 77 -), 
i. e. from — 35 to + 86*9, we have a total of 121 2 for the extent over which di- 
spersion scatters the sun’s rays, reckoned on a scale of which the Newtonian coloured 
spectrum occupies 53’8 parts, or considerably less than one half : and that this is not 
yet the full extent of prismatic dispersion is highly probable, since the insulation of 
the terminal thermic image can hardly be considered in any other light than as a result 
of absorbent action in one or all the glasses through which the rays have passed. 
Note II. — On a definite limit of Photographic Action of the Blue Rags under the in- 
fluence of an Iodic Salt. 
Received and read with Note I. 
129. A spectrum was formed on paper (No. 606.) prepared with alternate washes 
of nitrate of silver and common salt; and this action was continued till the impression 
became very intense. Without dismounting it, or in any way displacing the spectrum 
from exact coincidence with the photographic image, it was washed under the continued 
action of the light, with a moderately dilute solution of hydriodate of potash. The 
action immediately began to be reversed at the more refrangible end, and as far as 
the junction of the blue and indigo rays, or rather, to speak more distinctly, as far 
as + 25 0 in the actual, or -f- 2T4 in the linear spectrum. After continuing the action 
a short time, the whole of the more refrangible end of the photograph, up to this 
limit, was eaten away, the limit being marked by a tolerably sharp and well-defined 
semicircular outline, while below, the effect was hardly to deteriorate the impression 
produced. (The actual spectrum was produced to the meeting for inspection). 
Note III. in addition to the foregoing Paper . 
Account of a Process for rendering visible the Calorific Spectrum by its effect on Paper 
properly prepared, and of some further results obtained respecting the distribution 
of Heat therein. 
Received March 12, — Read March 12, 1840. 
130. It is well known to artists in water colours, that their tints, when freshly laid 
on and wet, are deeper and darker than they ultimately become on drying, a change 
which must be allowed for in the colouring, or the effect will be spoiled. This is re- 
