of the optic nerves. The consideration of a particular species of blindness, however, has led Dr Wollaston to a somewhat dif ferent distribution of the optic nerves. After fatigue, arising from four or five hours' violent exercise, Dr Wollaston was affected by a partial blindness, of which he first became sensible by seeing only half the face of a person near him, and next by seeing only the termination "son," of the name "Johnson;" this blindness was to the left of the point of vision in each eye; it was not perfect darkness, but merely a dark shade; and in about fifteen minutes, it gradually passed off, in an oblique direction upwards towards the left. As it was referrible to an affection of the nerves, Dr Wollaston did not apprehend or experience any return of it, other nervous affections being produced by fatigue. Some years afterwards he again experienced this singular kind of blindness, without any obvious cause, and first became sensible of it likewise by seeing only the half of a person's face; but in this case the right side of both eyes was affected, and complete vision was suddenly restored by the joy produced on receiving information of the safe arrival of a friend from a hazardous enterprise. Dr Wollaston has a friend who has experienced the same affection for seventeen years past, whenever his stomach is considerably deranged; another friend was attacked by pain at the left temple, and at the back of the left eye, which was succeeded by this sort of blindness on the right side of each eye; he can see to write, see the paper he is writing upon, and the pen he writes with,-but not the hand that guides the pen. The affection in this case, Dr Wollaston fears, is a permanent one; the pain first experienced, seems to have arisen from some effusion causing a degree of pressure on the brain, and the blindness from the continuance of this pressure on the left thalamus nervorum opticorum.

Now all these cases seem referrible to the partial insensibility of each retina, and they indicate that the left side of the retina in each eye is supplied with nerves from the same thalamus, and the right from the opposite thalamus; so that the nerves supplying the former alone decussate, and not those of the right side; an arrangement which Dr Wollaston calls the semi-decussation of the optic nerves.

Dr Wollaston proceeded to illustrate this statement of the distribution of the optic nerves, from that observed in those of fishes in the sturgeon the eyes are diametrically opposite to each other, each on one side of the head, the left eye being entirely supplied with nerves from the left thalamus of the brain, and the right eye entirely from the right thalamus.

The blindness above described, Dr Wollaston remarked, does not appear to be rare, but is seldom particularly noticed, like many other things, because it is not understood.

This very interesting paper concluded with a short section in which Dr Wollaston applies the sympathy of structure in the eyes, indicated by the effects just noticed, to the explanation of the long agitated question respecting the cause of single vision with two eyes. Every point in each eye is supplied with a pair of filaments from the same nerve, and the two eyes thus sympathize with each other in every point; hence arises single vision; and hence also the reason why infants direct both eyes in a corresponding direction, instead of squinting.-[Ann. Philos.]

Nautical Magnetic Premium.-The Board of Longitude have conferred the parliamentary premium of five hundred pounds on Mr Barlow, of the Royal Military Academy, for his method of correcting the local magnetic attraction of ships. The great quantities of iron employed at this time in the construction and equipment of ships of war, produce so much deviation in the compass (varying according to the direction of the ship's head) as to render it almost an useless instrument in certain situations, particularly in high northern and southern latitudes. It appears by Lieutenant Foster's report of experi ments made in his majesty's ship Conway, under the superintendence of Captain Basil Hall, to latitude sixty-one degrees south; and under that of Captain Clavering, in the recent voyage of the Griper to latitude eighty degrees north, that the difference in the bearing of an object with the ship's head at east and west, amounted to twenty-eight degrees before the latter vessel left the Nore; this difference afterwards amounted to fifty degrees at the North Cape, and to seventy-five degrees at Spitzbergen. Great, however, as this effect was, the method recommended by Mr Barlow was completely successful in counteracting it. This is extremely simple; it consists in merely placing a small plate of iron abaft the compass, in such a direction as to counteract in any one place the effects of the other iron in the ship; after which, without removing it, it continues to do the same in all parts of the world, whatever change may take place in the dip or intensity of the magnetic needle. Three important advantages will result from this discovery. It will add greatly to the safety of vessels in dark and blowing weather; it will tend to the general correction of our charts of variation; and will dispel nine out of ten of the supposititious currents, so liberally supplied by navigators to account for every remarkable disagreement be

tween reckoning and observation, and of which there can be no doubt, the greater number have arisen from this longneglected error in the compass.-[Philos. Mag.]

Nature of the luminous Power of the Glow-worm, Fire-fly, &c.At a late meeting of the Royal Society, a paper on the nature of the luminous power of some of the lampyrides, the glowworm, fire-fly, &c., by Dr Todd, was read. This paper commences with some general remarks on the various causes to which the luminosity of the lampyrides has been ascribed; the explanation of Macartney and Macaine, that the light they emit is a simple product of vitality, being considered as the true one. Dr Todd then proceeds to a minute account of the apparent source, and characters of the light in the several animals; describing the manner in which its emission is affected by solar and other light, by heat, and by certain chemical agents respectively. In the lampyris splendidula, or glow-worm, the light is of a fine topaz yellow colour, with a tinge of green, and is extremely vivid within the compass of a few inches, but does not extend its brilliancy far around; within that space the hour may be seen on a watch by its means. The light of the fire-fly is a pale yellowish tint, with continual flashes of vivid light; its variations are not connected with the motions of the insect's wings, nor are they produced, as some have affirmed, by the frequent intervention of a membrane. This animal may be seen shining in full moon-light; which is not the case with its congeners. Irritants excite the luminous power in all cases, and disorganizing substances destroy it. Dr Todd concludes that this power is solely an effect of vitality, and that the light may be considered as animal light; being analogous to animal heat, which arises from a power of separating heat from its combinations with matter. He adopts the hypothesis that its principal use is that of guiding the male insects to the female: the males always approach any light; and sometimes even the shining females of other species, until they come very near them. The fact that the larvae, and even the ova, possess a degree of the luminous faculty, Dr Todd does not consider as militating against this explanation; for various organs are partially developed in the earlier stages of many animals, which are only to be used by them when arrived at their perfect state.-[Ann. Philos.]

Igneous Meteors.-At a late meeting of the Meteorological Society of London, a paper, by Mr Brayley, was read, on the phenomena of igneous meteors; in the course of which, the phenomena attending the fall of the meteorites at Noble

borough, in the state of Maine, described in vol. i. page 386 of this Journal, were noticed. Mr Brayley pointed out a remarkable affinity, in mineralogical characters, subsisting between these meteorites, and those which fell respectively, at Loutolox in Finland, in 1822, at Jonzac, in France, in 1819, and at Juvenas, in the same country, in 1821. This affinity partly consists in the strong resemblance which they all bear to certain products of volcanoes; whilst the meteorites of several other descents connect them, by a gradual transition, with those whose characters are more peculiar. From these and other circumstances, in conjuction with that of the frequent presence of olivine in meteorites, the author infers, that the agencies which give rise to volcanic phenomena, whatever these may be, and however exerted in this case, are probably concerned in the production of igneous meteors, and the bodies which descend from them.

Silicon.-Professor Berzelius has succeeded in obtaining the base of silica in an insulated state. The method consists in acting by potassium on dry silicated fluate of potash, by which means a mixture of various substances is obtained, which yields hydroguret of silicon by being well washed with water; and when that substance is heated in a crucible the hydrogen is burned off, and the silicon obtained pure. Professor Berzelius has given the results of various experiments upon this substance; among which are the following. It is obtained in various states of aggregation, and its combustibility varies accordingly, it much resembling carbon in this respect; as usually obtained, it is combustible when ignited in atmospheric air, and in oxygen gas; but in its densest state, it may become incandescent in the air without burning. It is very difficult to effect its complete combustion; 200 parts of silicon unite to 208 of oxygen to become silica. It will not burn when heated with nitre, but is brought into combustion by carbonate of potash; a curious circumstance, which the author attributes to certain relations of affinities. Silicon burns when ignited in chlorine, forming with it a transparent. colourless fluid, having the smell of cyanogen. It is combustible in vapour of sulphur, producing a grey sulphuret, but cannot in this case be completely burned.-Ann. Philos., June.]

New Minerals.-Mr Brooke has lately described two new minerals; to one he has given the name Childrenite, in honour of Mr Children, who has contributed so much to mineralogical chemistry; the other he has called Somervillite. Childrenite was met with in Devonshire, and was at first supposed to be carbonate of iron, but Dr Wollaston determined that it was

phosphate of alumina and iron. The primary form of the crystals is, according to Mr Brooke, a right rhombic prism. The crystals scratch glass, and their colour is wine yellow, resembling some varieties of sulphate of barytes.

Somervillite was sent, among other Vesuvian minerals, to Mr Brooke, by Dr Somerville; the primary form of the crystals is a right square prism; colour a very pale dull yellow; they occur in cavities with crystallized black mica. This substance might at first view be mistaken for idocrase; but it is much softer, the cleavage parallel to the terminal planes much more distinct, and the cross fracture more glossy. Mr Children has compared the characters of this mineral under the blow pipe with those of idocrase. When exposed alone in the forceps, it slightly decrepitates, which idocrase does not, and fuses, with greater difficulty than idocrase, into a greyish glass, the globule from idocrase being greenish. With borax, in the reducing flame, idocrase produces a light-green, and this a colourless glass.

Mr Brooke has also ascertained that the mineral called kupferschaum by the Germans, is the same as the fibrous or flaky bright-green substance found at Matlock, and is a carbonate of copper and zinc.

Iodine in Mineral Waters.-M. Angelina has discovered iodine in the waters of Sales, in Piedmont; and M. Krüger, of Rostoch, has detected it in the mother liquor of the saline springs of Sülzer, in Mecklenburgh-Schwerin. M. Fuchs has also found the same substance in the mother water of the Sal Gem of Halle, in the Tyrol. It appears, however, that as yet the iodine has not really been separated from their mother liquors, but its presence has been ascertained by the blue colour given to starch dissolved in nitric acid, and there appears to be no doubts in the minds of the experimenters, on the reality of its presence.-[Gior. de Fisica.]

South America.-M. Humboldt communicated to the Academy of Sciences at its last sitting, some very interesting observations made by Messrs Boussingault and Reveno. These gentlemen have analyzed an aërolite, weighing seve ral quintals, which they found in the mountains of Santa Rosa, to the northeast of Santa-Fe-de-Begota. They say, according to letters from Antioquia, that a mass of native gold has lately been found, weighing more than one-hundred and ninety pounds. They have proved the presence of both sulphuric and muriatic acids, in the waters of a small river which runs from the volcano of Paracé, near Popayan,and which is called by the inhabitants Vinegar river.-[Philos. Mag.]