LIGHT AND RADIANT HEAT. 
253 
to undertake this double enquiry. The premature death which 
has deprived us of our colleague, has also deprived us of the 
great information which he would, no doubt, have obtained on 
this subject j as he has already shewn in his excellent disco- 
veries on other parts of optics. But in this respect, at least, 
the researches he had commenced or projected have not been 
lost. M. Berard has pursued them with great care , and hav- 
ing terminated them with all possible accuracy, he has presented 
you with the results. 
With regard to apparatus, M. Berard has had great advantage Heliostat used 
beyond the philosophers who have preceded him in his re- Bera *d 
J r r r for the new 
searches. He made use of the heliostat which Malus had experiments* 
caused to be constructed for the philosophical cabinet of M. 
Berthollet, and by means of this instrument he obtained a solar 
ray perfectly fixed, upon which he could operate at pleasure. 
By decomposing this ray of light with a prism, he obtained a IVTotionless 
coloured spectrum, which was motionless \ and by placing very spectrum, 
delicate thermometers in the spaces occupied by the different 
colours, he was enabled to compare their calorific properties 
with the greatest certainty. He determined their chemical pro- 
perties by placing, instead of the thermometers, such chemical 
compounds as were readily susceptible of alteration. 
He first examined the calorific qualities of the different rays j The max j mum 
they are known to be unequal. M. Rochon, who first observed of radiant heat 
this inequality, placed the maximum of heat in the yellow treme redTbut 
rays, where the property of enlightning is also the strongest. not be y°od it. 
M. Herschel fixed it outside the spectrum, beyond the ex- 
treme red rays. The experiments of M. Berard have confirmed 
those of Herschel relative to the progressive augmentation of 
the calorific quality from the violet to the red j but he found 
the maximum of the heat at the extremity of the spectrum itself, 
and not beyond it. He fixes it at the point where the ball of 
his thermometer was still entirely covered with the red rays, 
and he saw that the temperature was gradually less and less, as 
the ball of the thermometer was removed into the obscurity. 
Lastly, by placing the ball of the thermometer entirely out of ^ ^ ^ ^ 
the visible spectrum, at the place where M. Herschel has fixed extr eme S point 
the 
