604 
the quartzo-dolomitic series the amount of metamorphism, though 
the materials are not favourable for its production, is consider- 
able ; and the rock has a general resemblance to some of the 
impure calcareous bands which are incorporated with true schists 
in the Alps. 
Further, although our knowledge does not at present enable 
us to speak dogmatically on this point, the weight of evidence 
is, In my opixion, strongly against the probability of the Newer 
Gneiss series being altered Silurian rock. I would even go so 
far as to say that it is such as to throw the onus probandi on 
those who assert its (comparatively) modern date. For five or 
six years I have been working—I trust without prejudice—at the 
question of the age of metamorphic rocks, during which time I 
have visited typical districts in Cornwall, Wales, Scotland, and 
the Alps ; and in every case have been driven to the same con- 
clusion, namely, that wherever extensive regional metamorphism 
exists, the antiquity of the rocks is very great, so that they are ; 
probably anterior to the Cambrian period. I fully expect that 
when the Durness region is closely scrutinised, it will be found 
that this fossiliferous limestone is faulted down against the meta- 
morphic series, exactly (for instance) as the so-called Devonian 
rocks of the Lizard are faulted against the ‘‘ hornblende schists” 
of that district, and are a remnant, thus preserved, of a more 
modern and wide-spread series. Any geologist who would 
setile this point for us would be entitled to our gratitude, but to 
do it will require no ordinary conjunction of qualifications ; for 
he must bea practised microscopist, a skilled worker in the field, 
and_a man who cares for truth more than for the traditions of an 
office, or even his own preconceived opinions. 
23, Denning Road, Hampstead T. G. BONNEY 
WITH regard to Mr. Hudleston’s letter on the aboye subject, 
published in NaTuRE (vol. xxv. p. 582), I am glad to say that 
I am still alive, and able to give a part, at least, of the desired 
evidence for connecting the Durness limestone with the rocks of 
Assynt ond Erribol. 
In the year 1858 I accompanied Sir Roderick Murchison, 
while on a geological tour in Sutherland. During our stay at 
Inchnadamff, one of our excursions led us together up the River 
Traligill, Opposite the place where the springs issue from the 
miniature limestone caverns, about two miles above the bridge, 
I espied the fossils in dispute— ‘‘ orthoceratites — partially 
weathered out of the dolomitic limestone from which the stream 
issues. So overjoyed was I, that I called Sir Roderick to my 
side by shouting “ Eureka,” as I was a little in advance of him, 
pointed out the fossils 77 sztv, and after hammering them out of 
their bed, handed them to him. The circumstances of the 
achievement are indelibly impressed on my memory. 
saw these fossils in the field, I am not able to tell to what species 
they belonged ; but there can be no doubt of their nature, as in 
my attempt to hammer them out of the rock, one of them was 
broken in such a manner as to expose the septa and the 
siphuncle. 
On a subsequent visit which I made to Sutherland, I had the 
good fortune to see the specimen of Orthoceras (Cameroceras) 
Lrongniartit alluded to by Mr. Hudleston as ‘‘having been 
found in the upper quartz-rock of Erribol.” It was in the 
possession of the finder, the late Mr. Clark, of Erribol House, 
who kindly allowed me to examine it. Mr. Clark accompanied 
me to the place, and pointed out the exact spot where he got 
the specimen—a little to the north-east of Erribol House. 
Cuas, W. PEACH 
30, Haddington Place, Edinburgh, April 24 
The Magnetic Storms 
THE magnetographs at the Kew Observatory were a little 
disturbed from about 11 p.m. of the 13th inst. to 7 p.m. of the 
14th inst. During the r5th they were quiet, and remained so 
up to 11.45 p.m. of the 16th, when the disturbance began by an 
increase of the declination, an augmentation of the horizontal 
force, and a diminution of the vertical force. The movements 
of the declinometer became gradually more rapid after 2 a.m. 
on the 17th, whilst its oscillations extended farther and farther 
from its normal position principally in the direction of increased 
westerly declination. 
From 4.30 to 9 a.m. the horizontal foree had diminished so 
much that the trace frequently passed off the paper and the 
register was lost fora while. At this time the force must have 
been more than ‘05 mm.mgrs, below its average value. 
NATURE 
As I only 
The minimum of vertical force occurred at 5.55 a.m., when 
it was about 0°07 units too low. 
From 10 a.m. to noon of the 17th the motion of the declino-- 
meter was small, whilst the components of magnetic force were 
rapidly increasing in intensity, until at 0.15 p.m. both traces Jeft 
the photographic sheet in the direction .of augmented force ; at 
this time the declination needle merely oscillated rapidly about 
its ordinary position. 
The horizontal force instrument recommenced to record about 
2p.m., and the vertical force about 2.45 p.m.; afterwards the 
movements of all three gradually diminished, and at about 
8 p.m. the disturbance had died out. 
During the 18th and 19th the magnets remained unaffected, 
but at 3.45 a.m. of the 20th a second disturbance set in, com- 
mencing with a rapid increase of declination, the first swing of 
the magnet carrying it nearly a degree to the westward, whence 
it returned at 4.30 a.m. 
a.m., and then its oscillations became very rapid, and continued 
so until 2 p.m ,lafter which‘hour they became less; but the effect 
of one disturbance lasted until 7.30 a.m. of the 21st. 
Both forces were also simultaneously disturbed, but their 
movements were much more limited than on Monday, the ex- 
tremes being in the horizontal og mm.mgrs., and in the vertical 
o°3 mm.megrs. only. G. M. WHIPPLE 
Kew Observatory, April 24 
Colour Perception 
WHILE working at dry-plate photography in a ruby light, 1 
noticed that when any light-coloured article, such as the hand, 
was rapidly moved, it appeared of a brilliant greeni-h-blue, 
in which blue predominated, while, when slowly moved, it 
appeared of the same colour as the other objects in the room. 
Seeking for an explanation, led me to recognise a new fact about 
colour perception which may be of interes to your readers. The 
reason of the hand appearing blue when in rapid motion was 
because the continual use of the red light had fatigued ,that part 
| of the retina responsive to it, and the light reflected from the 
hand impinging for a very short time on the retina, was not 
strong enough to excite the sensation of red, but was quite suffi- 
cient for blue, the nerves responding to this colour having been 
rendered acutely sensitive by complete rest. To test this hypo- 
thesis, I obtained some dark blue glass and applied it to the 
window of the dark room, removing the red. On repeating the 
experiment, the eye with its blue sense exhausted, saw rapidly- 
moving objects reddish. Now from this it is clear that it takes 
a longer time to cause a sensation in an exhausted than in a 
fresh organ. It also gives a direct proof of Helmbholtz’s sugges- 
tion, ‘‘that actual coloured light does not produce sensations of 
absolutely pure colour; that red, for instance, even when com- 
pletely freed from all admixture of white light, still does not 
excite those nervous fibres alone which are sensitive to impres- 
sions of red, but also to a very slight degree those which are 
sensitive to green, and perhaps to a still smaller extent those’ 
[April 27, 1882 
caeveer is ware 
Its mean position was reached at 6— 
which are sensitive to violet rays” (‘‘ Popular Scientific Lec- - 
tures,” first series, p. 223). These observations have led me to 
an explanation of a very curious phenomenon brought under my 
notice by my friend, Mr, Napier Smith. When discs of paper 
on which black spaces have been marked, so that on rotation the 
eye receives impression: of black and white too rapidly to notice 
the pattern, but too slowly to combine into a neutral gray, the 
rotating card appears to be distinctly coloured, especially when 
it is looked at without keen attention, or as we may say 
passively. All colours may be seen, but red and blue were the 
most distinct to me. I at first thought that the colour might 
arise out of the paper and ink, the former being perhaps tinted 
with blue to whiten it in manufacture, and the latter probably 
a dark brown; but on looking several times at the solitary 
discs, and acquiring the power of looking passively the intensity 
of the colours could not be so accounted for. The true 
explanation is found, I believe, in the fact that the different 
colour organs require longer or shorter periods of excitation 
before responding to the stimulus, and that those which require 
the longest periods also retain the sensation longest. I have 
only made very rough trials, but they point to the fact that the 
eye responds quickest to red, so that the most rapid alternation 
will appear reddish, a little slower green will come in, and cause 
some indescribable colours, such as are seen in the polariscope, 
and lastly, when green and red are about equal, and producing 
| white, blue will be seen. ‘The blue is best seen with a slo. 
