rpose the readings of the spectroscope are
reduced to wave-lengths by means of interpolation curves; or if Zeiss's
microspectroscope be used, the position of bands in wave-lengths
(denoted by the Greek letter [lambda]) may be read directly.

Haemoglobin, the red colouring matter of vertebrate blood,
C758H1203N195S3FeO218, and its derivatives haematin, C32H30N4FeO3, and
haematoporphyrin, C16H18N2O3, are colouring matters about which we
possess definite chemical knowledge, as they have been isolated,
purified and analysed. Most of the bile pigments of mammals have
likewise been isolated and studied chemically, and all of these are
fully described in the text-books of physiology and physiological
chemistry. Haemoglobin, though physiologically of great importance in
the respiratory process of vertebrate animals, is yet seldom used for
surface pigmentation, except in the face of white races of man or in
other parts in monkeys, &c. In some worms the transparent skin allows
the haemoglobin of the blood to be seen through the integument, and in
certain fishes also the haemoglobin is visible through the integument.
It is a curious and noteworthy fact that in some invertebrate animals in
which no haemoglobin occurs, we meet with its derivatives. Thus haematin
is found in the so-called bile of slugs, snails, the limpet and the
crayfish. In sea-anemones there is a pigment which yields some of the
decomposition-products of haemoglobin, and associated with this is a
green pigment apparently identical with biliverdin (C16H18N2O4), a green
bile pigment. Again, haematoporphyrin is found in the integuments of
star-fishes and slugs, and occurs in the "dorsal streak" of the
earth-worm _Lumbricus terrestris_, and perhaps in other species.
Haematoporphyrin and biliverdin also occur in the egg-shells of certain
birds, but in this case they are derived from haemoglobin. Haemoglobin
is said to be found as low down in the animal kingdom as the
Echinoderms, e.g. in _Ophiactis virens_ and _Thyonella gemmata_. It also
occurs in the blood of _Planorbis corneus_ and in the pharyngeal muscles
of other mollusca.

A great number of other pigments have been described; for example, in
the muscles and tissues of animals, both vertebrate and invertebrate,
are the histohaematins, of which a special muscle pigment, myohaematin,
is one. In vertebrates the latter is generally accompanied by
haemoglobin, but in invertebrates--with the exception of the pharyngea