452 
NATURE 
[ Oct. 6, 1870 
meteorological observations. The weather tables now pub- 
lished in the daily papers are of comparatively little value 
to the general public, owing to their want of arrangement. 
A weather-table, to be of any great practical value in the northern 
hemisphere, should contain—first, a record of observations 
made at points south of the place for which the table is designed, 
and if possible on or near the equator and the 3oth to the 32nd 
parailel ; next, a record of observations made at western points 
within the limit of what I have ventured to call the storm area ; 
next, a record of observations at northern stations about latitude 
60°; and next, at eastern stations within the storm area. This 
statement is not to be understood as implying that all storms 
begin at the equator or at any one point of the compass, or that 
they are in all cases confined within one section of the atmo- 
sphere, or move in one direction. 
Fourth :—It is very important to obtain correct and 
copious data regarding the atmospheric currents between 
(say) 5,000 feet and five miles above the level of the sea, 
and especially at various points on and near the equator, 
and at about 30° to 32° North and South latitudes. Within 
these limits the rain-bearing currents of the atmosphere move. 
If self-registering meteorological instruments were placed 
permanently upon several of the leading mountain ranges of the 
world, and their records copied at stated intervals, we should 
obtain valuable data for determining the direction, velocity, and 
magnitude of the controlling atmospheric currents of the globe, 
More valuable still would be data obtained by the use of self- 
registering and self-regulating machines that would ascend to any 
desired height within the limit mentioned above, remain up fora 
time determined by clock-work, and then descend, bringing with 
them complete records of temperature, moisture, direction and 
yelocity of currents, &c. Additional facts regarding the great 
atmospheric currents within the limits named are required to 
enable us to interpret correctly the oscillations of barometers near 
the surface of the sea. G 
New York 
Colour Blindness 
I HOPE your readers will bear with a few more observations 
from me on the foregoing subject. It is an undoubted fact that 
modes of nervous action which have once coexisted tend to ex- 
cite each other afterwards, This the phenomenon known as 
Association of Ideas sufficiently proves. It is to this cause that 
I should be disposed to attribute the phenomenon of accidental 
or complementary colours. Ido not, of course, mean that this 
Jatter fact can be resolved into Association of ideas, but that 
it and association depend on the same organic law. It is 
robable that in addition to this, however, the mutual excitation 
of vibrations comes in. Judging from the determination of 
wave-lengths by diffraction, the ratio of the wave-length of any 
given ray to that of the complementary one is not very far from 
2 to 3 or 3 to 2; and, as the colours excited do not seem to be 
exactly complementary, it is probable that the vibration referred 
to is that chiefly excited. For an accurate determination of this 
question we should require to determine the wave-lengths for 
each colour in the liquid whjch surrounds the eye, or rather, 
perhaps, in the retinal substance, Such are the two causes to 
which I shculd be disposed to ascribe the phenomenon of acci- 
dental colouis, From the former it would follow that the more 
completely the eye had been accustcm ~ to white light, the more 
likely the person would be to become co'our blind ; and that 
colour-blindness might be remedied, or at least altered in 
character, by accustoming the eye to look at everything through 
coloured glasses. 
There are, however, subjective causes which determine the 
eye to exhibit various colours in particular instances, OF these, 
jaundice, which makes the object appear yellow, is one. De- 
rangement of the stomach, I believe, often invests objects in the 
dark with a blue tint. Colours of this description constantly 
mingle with our ordinary perceptions without being noticed, but 
when a finer distinction is needed it becomes necessary to 
avoid them. Thus, in some of the delicate optical experiments 
to which I referred in my last, the experimentalist must not 
operate after a long fast or a hearty meal, or after taking any 
alcoholic drink. He sometimes finds, also, that his two eyes 
differ in their appreciation of colours. Two instances of this sub- 
jective colouring recently came under my notice. In one case 
an old lady for some time Saw everything red, which she attri- 
buted to looking very much at a red flower (which she greatly 
admired) in her sitting-room. She was advised to look a good 
deal at the green fields opposite her window, and soon recovered. 
In the other case a friend of mine, reading a note-book which 
he had marked with a blue pencil, was surprised to find that the 
marks appeared green. He showed me the book, and themarks 
were quite unaltered to my eye. He was reading hard, and 
somewhat nervous, but otherwise in good health. In most of 
the cases I have referred to, the subjective colouring was only 
temporary ; but I have little doubt that there frequently exists 
a permanent subjective colouring which modifies all the pheno- 
mena of vision, and leads to effects, in some respects, similar to 
those of ordinary colour-blindness. 
Some time ago the question was suggested to me whether a 
blind man (whose retina was not destroyed) could have a percep- 
tion of extension by sight, or whether, in the uniform darkness 
which was supposed to surround him, he could distinguish exten- 
sion at all? 1 tried the experiment by shutting my eyes, but 
finding that when the light was good a very perceptible amount 
made its way through the eyelids, I placed bandages over them 
in addition. I could never, however, obtain uniform darkness. 
Points over the field always appeared less dark than the sur- 
rounding ones, and the positions of these points were easily 
distinguishable. On making the experiment for the third or 
fourth time, I was rather surprised to see part of the 
visible sur'ace covered by a faint blue. Subsequent ob- 
servation confirmed this, and I now believe that it was only 
from habitual inattention to colours when the eye was shut (as well 
as the difference between colours on a dark and on a bright 
ground), that I failed to observe it on the first occasion. These 
colours are by no means confined to blue. In fact, I think I 
have seen every tint in the spectrum, and I never can close my 
eyes for three minutes without seeing some of them more or less 
distinctly. They generally only cover a patch which, however, 
is not fixed, and the colour first visible sometimes disappears 
altogether. I may, perhaps, add to this that I often see a col- 
lection of small, bright, round moving spots about the centre of 
the field of vision. These spots I can likewise see in the dusk 
with my eyes open. This affords a proof of subjective colours 
in my own eyes, but from their variability, I do not think my 
eyes have a predilection for any particular tint. I often see 
more than one of these colours at the same time. If the eye 
had a predisposition for ary particular colour, it could probably 
be discovered in this way. The observations are worth repeat- 
ing. A close attention to the sensations is required on account 
of their faintness. 
I have mentioned the expedient of Jooking through coloured 
glasses. Professor Wartmann succeeded in this way in making 
colour-blind persons distinguish colours which they confcunded 
with the naked eye—a fact quite explicable on my principles ; 
but he found it impossible to predict what effect a glass of any 
given colour would produce. But might we not by repeated ex- 
periments hit upon the particular tint which would suit the eye 
of any particular patient? If, for example, we tried his eye with 
a solar spectrum, and interposed one or more coloured glasses of 
different tints, and in this tentative manner at last succeeded in 
making him see seven colours, each confined within the same 
limits that they are to the ordinary eye, it is pretty evident that 
these glasses would enable him to distinguish the colours of all 
objects in the daylight. 
Not having read much on the subject, I cannot say how far 
these views are original. The subject is at least one which re- 
quires further investigation, and if I can induce some of the 
eminent contributors to your columns to take up the matter, my 
letters will not have been vain though the result may be to 
overthrow all that I have advocated, W, H, S, Monck 
Trinity College, Dublin 
The Effect of Tannin on Cotton 
IN your last number you mention the fact that cotton fabrics 
are rendered more durable by treatment with tannin, as if it was 
a new discovery, and state that ‘‘it is believed the change 
cannot be great, since it has escaped the notice of practical 
tanners.” 
In our neighbourhood (the coast of Northumberland) the 
fishermen have, for many years past, been in the habit of tanning 
their nets and sails with oak-bark or catechu, 
