Biographical Memoir of Sir William Herschel. 819 
understand how the quantity of faint light from the steeple, 
that passed through the pupil of the unassisted eye, should be 
incapable of performing the functions of vision, while a quanti- 
ty of the same light, falling on a speculum twelve inches in dia- 
meter, was perfectly sufficient to render the same object dis- 
tinctly visible. This interesting subject Dr Herschel has treated 
with great perspicuity in his paper “ On the Power of penetrating 
into Space by Telescopes , with a comparative determination of 
the extent of that power in natural vision , and in telescopes of 
various sizes and constructions ,” which was published in the 
Philosophical Transactions for 1800. 
Hitherto we have followed our author in his varied excur- 
sions among the bodies of the solar system, and among the in- 
tricate phenomena of the starry heavens ; but we shall now 
trace him into a new field of discovery, which he has render- 
ed one of the most fertile and interesting in general physics. 
While engaged in experiments on the most advantageous me- 
thods of viewing the sun with large telescopes, he had occasion 
to employ various combinations of differently coloured darkening 
glasses. With some of these he felt a sensation of heat when there 
was very little light, while others gave him much light with scarce- 
ly any sensation of heat. The sun’s image being in these cases 
differently coloured, it occurred to Dr Herschel, that the pris- 
matic rays might have the power of heating in a very unequal 
degree ; and he immediately projected a series of experiments 
on that subject. He found that the heating power of the rays 
of the prismatic spectrum, increased gradually from the extremi- 
ty of the violet space to the extremity of the red space, where- 
as the power of illuminating objects was a maximum in the yel- 
low rays, and decreased towards both extremities of the spec- 
trum. In measuring the heating power of the red rays, he was 
surprised to find that the thermometer still rose when it was 
placed a little without the red extremity of the spectrum, and 
he was thus led to the curious and important discovery of in- 
visible rays beyond the red space , which had the power of heat- 
ing but not of illuminating objects. The invisible heat exerted a 
considerable power over the thermometer even at a point 1 \ inches 
beyond the extreme red, when the distance of the spectrum from 
the prism was 58 inches. 
