e, of a sunbeam. The effect of passage through a prism is to
crowd together the red rays very much more than the blue. To this
prismatic distortion was owing the establishment of a pseudo-maximum of
heat in the infra-red, which disappeared when the natural arrangement by
wave-length was allowed free play. Langley's bolometer has shown that
the hottest part of the normal spectrum virtually coincides with its
most luminous part, both lying in the orange, close to the D-line.[735]
Thus the last shred of evidence in favour of the threefold division of
solar radiations vanished, and it became obvious that the varying
effects--thermal, luminous, or chemical--produced by them are due, not
to any distinction of quality in themselves, but to the different
properties of the substances they impinge upon. They are simply bearers
of energy, conveyed in shorter or longer vibrations; the result in each
separate case depending upon the capacity of the material particles
meeting them for taking up those shorter or longer vibrations, and
turning them variously to account in their inner economy.

A long series of experiments at Allegheny was completed in the summer of
1881 on the crest of Mount Whitney in the Sierra Nevada. Here, at an
elevation of 14,887 feet, in the driest and purest air, perhaps, in the
world, atmospheric absorptive inroads become less sensible, and the
indications of the bolometer, consequently, surer and stronger. An
enormous expansion was at once given to the invisible region in the
solar spectrum below the red. Captain Abney had got chemical effects
from undulations twelve ten-thousandths of a millimetre in length. These
were the longest recognised as, or indeed believed, on theoretical
grounds, to be capable of existing. Professor Langley now got heating
effects from rays of above twice that wave-length, his delicate thread
of platinum groping its way down nearly to thirty ten-thousandths of a
millimetre, or three "microns." The known extent of the solar spectrum
was thus at once more than doubled. Its visible portion covers a range
of about one octave; bolometric indications already in 1884 comprised
between three and four. The great importance of the newly explored
region appears from the fact that three-fourths of the entire energy of
sunlight reside in the infra-red, while scarcely more than one-hundredth
part of that amount is found in the better known ultra-violet
space.[736] These curious facts were reinforc