OPTICAL LABOURS. 301 
beautiful comet of 1811, relative to the changes of dis- 
tance from the sun, and the modifications resulting 
thence, Herschel put it beyond doubt that these modifi- 
cations have something individual in them, something 
relative to a special state of the nebulous matter. On 
one celestial body the changes of distance produce an 
enormous effect, on another the modifications are insig- 
nificant. 
OPTICAL LABOURS. 
I shall say very little on the discoveries that Herschel 
made in physics. In short, everybody knows them. 
They have been inserted into special treatises, into ele- 
mentary works, into verbal instruction ; they must be 
considered as the starting-point of a multitude of im- 
portant labours with which the sciences have been 
enriched during several years. 
The chief of these is that of the dark radiating heat 
which is found mixed with light. 
In studying the phenomena, no longer with the eye, 
like Newton, but with a thermometer, Herschel discov- 
ered that the solar spectrum is prolonged on the red side 
far beyond the visible limits. The thermometer some- 
times rose higher in that dark region, than in the midst 
of brilliant zones. The light of the sun then, contains, 
besides the coloured rays so well characterized by New- 
ton, some invisible rays, still less refrangible than the 
red, and whose warming power is very considerable. A 
world of discoveries has arisen from this fundamental 
fact. 
The dark heat emanating from terrestrial objects more 
or less heated, became also subjects of Herschel’s investi- 
gations. His work contained the germs of a good num- 
