296 
SECOND REPORT— 1832 . 
which, though quite opake for light, may transmit heat co¬ 
piously. 
Sir D. Brewster considers Sir W. Herschel’s experiment on 
the refraction of {{ culinary” heat by lenses, to be very unsatis¬ 
factory, as before noticed. He recommends a lens composed 
of zones, so as to have no greater thickness in the middle than 
towards the edges, a construction which he has described 
in his “Optics,” p. 322 (Cabinet Encyclopl), and made of glass, 
which unites the highest refractive power, with the smallest ab¬ 
sorptive power for heat. 
It is also important to find, as sources of heat, bodies which 
do not become luminous till at extremely high temperatures. 
6.) The researches of M. Melloni have also been extended 
to this part of the subject. (Annates de Cliimie , Dec. 1831, 
p. 388.) 
From known observations on the spectrum, he remarks that 
there exists on opposite sides of the maximum, isothermal 
points ; one in a coloured part, the other without the red end 
of the spectrum. 
On causing the different rays to pass through a plate of water, 
and noting the effect on the tliermo-multiplier; the heat of 
the violet ray was undiminished, but its isothermal totally inter¬ 
cepted. 
That of the indigo slightly diminished; its isothermal not 
totally intercepted. 
Proceeding in this way with the other rays, he found in 
general that the portions of heating power intercepted in the 
coloured rays, and those which are transmitted in their iso¬ 
thermal rays, increase in proportion as they approach the 
position of the maximum, where of course upon the whole the 
interception is greatest; or, in other words, the rays of the 
calorific spectrum undergo an interception by water in pro¬ 
portion as their refrangibility is less. 
He gives a Table of the numerical results. He views his re¬ 
sults as precisely according with and explaining those of See- 
beck. With a water prism the heating orange and red rays are 
more intercepted than the yellow; in this therefore the maxi¬ 
mum appears. 
Conclusion. 
We have thus far taken as close a survey as is consistent with 
the limits of a Report like the present, of the successive and va¬ 
ried researches which have been made with the view of tracing the 
laws of radiant heat. In the present state of our knowledge, it 
must upon the whole be avowed, that we have little to contem- 
