By reason of his semi-tropical climate the Australian is bathed in an
atmosphere of sunshine. This has a distinct effect upon the blood, for
the action of sunlight upon this fluid is to redden it--a fact which
has for ages been dwelt upon by the poets. But for a scientific
explanation of this effect of sunlight in reddening the blood we must
turn to the spectrum analysis. The visible solar spectrum as shown
through a prism by the ordinary sunbeam is made up of the seven
different colours, namely, red, orange, yellow, green, blue, indigo,
and violet. Instead of consisting simply of white light as a whole, it
is now universally accepted that in this spectrum different properties
belong to different parts. Light or luminous power to one portion; heat
or calorific power to another; and chemical power or actinism to a
third.

The visible solar or Newtonian luminous spectrum, resulting from the
decomposition of white light by a prism, is only the middle portion of
the whole solar spectrum. Beyond the red end there are rays possessing
still greater-heating effect; and beyond the violet extremity there are
rays endowed with far more powerful chemical action. The violet, and
especially these latter ultra-violet rays, redden the life stream by
increasing the haemoglobin--that crystallizable body which forms so
large a portion of the coloured corpuscles of the blood. Sunlight,
moreover, has not only this action upon the animal kingdom, but also
upon the vegetable world as well Plants, like celery, which are
subjected to blanching, become whitened under the process of
etiolation. This is due to the absence of chlorophyll, the
green colouring matter of plants, which can only be developed by the
presence of light. The tops of celery, being unearthed, retain their
green colour, while the stem embedded in the soil acquires its familiar
whiteness.