58 
SIR J. F. W. HERSCHEL ON THE CHEMICAL ACTION OF THE 
the spots, and to that idea I rather incline. But the spectrum is so much contracted 
in length when a crown-prism is used, that it is not easy to say with certainty how 
far the apparent obliteration of the separating minima might be attributable to that 
cause. 
142. When the spectrum formed by the flint-prism and achromatic lens, as usual, 
was displaced laterally by refraction through the thick part of a crown-prism, but 
little alteration took place in the relative intensities of a, f3, and 7 , which experiment, 
so far as it goes, corroborates the conclusion of the last paragraph. 
143. A very different result was obtained by substituting a water-prism. The spot 
y was greatly enfeebled by this, nor could l be traced at all. The scale of transca- 
lescence of water, therefore, would appear to be considerably more contracted than 
that of either flint- or crown-glass. And the same conclusion was arrived at by in- 
terposing between the lens and its focus a parallel-sided box of plate-glass. It having 
been first ascertained that the box when empty allowed all the spots, a, /3, 7 , c), to pass 
freely, it was filled with distilled water, which completely destroyed both 7 and ^ ; 
and so far curtailed (3 , that it could no longer be recognised as distinct, the thermic 
action being terminated at — 24 by a sharp and well-defined oval outline. The upper 
portion of the thermic spectrum, extending above the yellow into the green and blue 
rays, was not attacked. That the spot 7 was produced when a water yrhm was used, 
must be attributed to the thinness of that medium at the edge. When the box was 
used all the rays had to pass through 1‘4 inch of water, that being the interior interval 
between the transmitting surfaces. 
144. When muriate of lime (a saturated solution) was substituted for water, the 
result was nearly as with pure water, the action being cut oft’ at — 26, but not quite 
so sharply. I ought also to notice that it was in this experiment that the first suspi- 
cion of the existence of s was suggested, by which it might seem that the absorptive 
action of this medium relaxes for the extreme thermic rays. Nitrate of silver (spe- 
cific gravity T200) and nitrate of lead gave the same results as water. The sides of 
my glass box being united with a resinous cement, I could not try the effect of al- 
cohol, ether, essential oils, &c. ; nor has time permitted any further extension of the 
inquiry in this direction. Colourless quartz transmits a, (3, and 7 freely. 
145. Green glasses cut off nearly the whole thermic spectrum. The oval outline 
of (3, which passes between (3 and 7 , as also the spot 7 itself, may, however, be traced. 
A standard red combination, which insulated the extreme red by the joint action of 
a cobalt-blue and a dark brown glass, cuts off both 7 and &, and reduces the thermic 
spectrum to two white spots, a and (3, of which the latter is greatly enfeebled. The 
upper edge of the spot a (which is greatly curtailed in both directions) somewhat 
overlaps the lower edge of the red image of the sun formed by this glass. All the 
more refrangible thermic rays are cut off by this combination, as they are by a cobalt- 
glass used singly, which latter, however, allows 7 to pass freely enough. 
146. A brown glass of moderate intensity transmitted a, (3, and 7 , the latter copi- 
