204 
NATURE } 
phyll, which occurs in considerable quantity in Osctlla- 
toria, but is relatively almost absent in green leaves, and 
would not be separated by the method employed by the 
author in making the preparation. Comparatively pure 
blue chlorophyll, prepared from olive AZg@ by the method 
described in my late paper, gives a spectrum free from 
absorption over the whole of the green and a considerable 
part of the adjoining blue. The close resemblance, and 
yet decided difference, between the spectra of the blue- 
green colouring matter obtained from the two above- 
named sources, did not escape the author’s notice, but 
the methods employed were inadequate to prove that 
both contained the same principal blue-green substance, 
mixed in one case with one, and in the other case with 
another colouring matter. I may here say that the rela- 
tive amount of blue and yellow chlorophyll differs very 
much in different classes of plants, and even in the same 
plant, when in different conditions, and the study of this 
variation leads to results of great interest in connection 
with vegetable physiology ; since, amongst other things, 
it proves that leaves normally very yellow are quite unlike 
those that have turned yellow in autumn, but analogous 
to those which are abnormally yellow owing to absence 
of light, as though the deficiency of chlorophyll were in 
both cases due to weak constructive energy; and the 
comparative absence of yellow chlorophyll in such abnor- 
mally weak plants, belonging to the highest classes, causes 
their colouring to approximate much more closely to that 
of those of much lower organisation. 
I must say that I object to the term chlorophyll being 
applied, as by the author, to a mixture of the various 
yellow substances belonging to the xanthophyll group. 
with one or both of the above-named green substances, 
The green colour of leaves is due to them, and they are 
both actually green, one a blue-green and the other a 
yellow-green, so that the terms blue chlorophyll and yel- 
low chlorophyll appear to me very appropriate. It would 
be better and extremely convenient to adopt some such 
word as endochrome, to express any mixture of coloured 
substances contained in the cells of plants, which has no 
reference to any particular tint of colour, 
The very materially different position of the chief ab- 
sorption-band of chlorophyll when in the leaves of plants 
and when in solution has been noticed by the author, and 
likewise the difference in its position when the chlorophyll 
is dissolved in different liquids. He attributes this en- 
tirely to the difference in the density of the liquid, and con- 
cludes that in the leaves the chlorophyll may be combined 
with or dissolved in some dense substance, The difference 
in the position of the bands of chlorophyll is very small com- 
pared with the difference seen in the case of some other 
colouring-matters, and by carefully studying the question 
I have come to the conclusion that the position of the 
bands does not vary directly with the density of the sol- 
vent, or with any other general property, but is so inde- 
pendent that it is desirable to look upon it as a special 
property, and to call it the absorption-band-raising power. 
The extent to which the bands are raised varies much 
according to the substance ; but, as an apparent rule, if 
the position is altered, they lie nearer to the blue end 
when the substance is dissolved than when in a free state. 
In accordance with this view of the subject, it appears as 
though in the living plants chlorophyll and various other 
colouring-matters exist in a free state, not combined with 
or dissolved in any wax, fat, or oil, with which, however, 
they often combine when the plant is boiled in water, and 
with which they are combined when a solution is evapo- 
rated to dryness, so that the spectrum of such a dried-up 
material may, and often does, differ most materially from 
that of the endochrome in the living plants. As an illustra- 
tion of the opposite case, I may refer to the spectra of yellow 
flowers, which often show that the endochrome is combined 
with, or dissolved in, a fat or oil. When not thus combined, 
the spectra are so different that the colouring-matter 
might be, and sometimes has been, looked upon as dis- 
tinct, before the true cause of the difference was known. 
The microscope alone could not decide this question, 
since visible granules might not be the free colouring- 
matter, and, on the contrary, it might be free, and the 
particles too small to be separately visible. 
: H. C. SORBY 
(To be continued.) 
7 
RECENT RESEARCHES ON THE PHYSIO- 
LOGICAL ACTION OF LIGHT 
HE arrangements by which the mind is brought into 
relation with the outer world are—(1) a terminal 
organ, such as the retina, or the intricate structures of the 
internal ear, or the touch corpuscles of Wagner, for the 
reception of impressions from without; (2) a nerve, 
endowed with a special sensibility peculiar to the sense 
for the conveyance of influences from the terminal organ 
to the brain ; and (3) a sensorium or brain in which, on 
receiving these influences, changes occur which give rise 
to the phenomena of consciousness. 
Nerves act, therefore, as conductors from the terminal 
organs to the brain, These terminal organs are specially 
fitted for the reception of specific stimuli, such as the 
vibrations of the ether, which, when received by the 
retina, induce a change which is transmitted to the 
brain, and gives rise to the sensation of light, or the con- 
densations and rarefactions of the air which cause sound. 
But though specially fitted for these stimuli, the terminal 
organs may be affected in other ways. For example, 
mechanical pressure on the retina produces a sensation of 
light, and many diseases affecting the auditory appara- 
tus by compression, cause agonising sensations of sound. 
The nerves in connection with the sense organs are 
termed nerves of special sense, because they are supposed 
only to convey influences which are derived from the 
special terminal organs with which they are connected. 
These nerves are, however, themselves not affected only 
by the special stimulus which affects their respective ter- 
minal organ. As is well known, the optic nerve is not 
affected by light—a fact easily demonstrated by Marriot’s 
experiment showing that the retina at the entrance of the 
optic nerve is insensible to light. 
The nature of the specific change produced on the ter- 
minal organs by the action of external stimuli has not 
hitherto been experimentally examined. Let us take the 
case of the eye. Numerous hypotheses have been ad- 
vanced. The action of light on the retina has been con- 
jectured to be a mere communication of vibrations, an 
intermittent motion of portions of the optic nerve, ar 
electrical effect, a heating effect, or a photographic effect 
like that produced by light on a sensitive surface, but up 
to this time there has been no experimental evidence in 
support of either of these views. 
The result of investigations made by Mr. Dewar and 
Dr. McKendrick, of Edinburgh, communicated to the 
Royal Society of Edinburgh, has been to show that the 
specific effect of light on the retina and optic nerve is 2 
change in the electro-motive force of these organs. They 
have been able to demonstrate this by the following 
arrangements :—The eye of a frog rapidly killed by 
pithing is dissected out of the orbit, so as to leave the 
sclerotic entirely free from muscle, and a portion of optic 
nerve intact. This preparation is placed on the cushions 
of the well-known arrangement of Du Bois-Raymond for 
collecting electric currents from animal structures, con- 
sisting of two zinc troughs, carefully amalgamated on the 
inner surface, and containing pads of Swedish filter-paper 
moistened with a solution of pure neutral sulphate of 
zinc. To protect the eye from the irritating action 
of the sulphate of zinc, thin films of sculptors’ 
clay, mixed with a weak solution of chloride of 
“s) . 7 . Py 
[ Fuly 10, 1873 
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