When Critics have their various thoughts exprefs'd, ART. 1. Philofophical Tranfactions of the Royal Society of London, for the Year 1799. Part 1. 4to. 182 PP. 8s. 6d. Elmly. NINE Papers, and the Meteorological Journal, kept at the apartments of the Royal Society, form the contents of this Part of the Philofophical Tranfactions; of which, we fhall give a concife account. I. The Croonian Lecture. apon the Structure of Nerves. F. R. S. Experiments and Obfervations According to the original inftitution of this Lecture, mufcu lar motion ought to have been the fubject of it; but this author excufes himself, by alledging the great and immediate connection which exifts between the nerves and the mufcles, and by obferving, that any knowledge refpecting the nerves will prove an important acquifition towards the investigation of mufcular motion. A BRIT, CRIT. VOL. XIV, JULY, 1799. The The obfervations which are contained in the prefent Lecture, were made upon the eye and the optic nerve; and the substance of them is as follows: An explanation of the illumination fo confpicuous in the eye of the cat, as well as of many other animals, when feen nearly in the dark, has been attempted two different ways; firft, it has been fuppofed that the illumination arifes from the fcattered external light, which is collected in the eye, and is thence reflected; fecondly, that there is a quantity of light generated in the organ itself. The experiments of Profeffor Bohn, of Leipfic, prove that, in perfect darkness, no illumination is to be seen in the eye of the cat, and the opinion of any light being generated in the eye itself is thereby entirely exploded; fo that the only particular which remains to be afcertained, is the manner in which the eye collects and reflects the scattered external light. In order to elucidate this fubject, Mr. Home attempted fome experiments on living animals; but meeting with confiderable difficulty in that mode of operating, he was forced to make his experiments after the death of the fubject. "In doing fo," fays he, " it was found that a strong light thrown upon the cornea illuminated the iris, as it had done in the living eye; but, when the cornea was removed, this illumination disappeared. The iris was then diffected off, and the lucid tapetum completely expofed to view, the reflection from which was extremely bright; the retina proving no obftruction to the rays of light, but appearing equally tranfparent with the vitreous humour and crystalline lens. From thefe experiments it appeared evident, that no light is generated in the eye, the illumination being wholly produced by the concave bright-coloured furface of the tapetum collecting the rays of the external light, concentrated by the cornea and cryftalline lens, and reflecting them through the pupil. When the iris is completely open, the degree of brilliancy is the greateft; but, when the iris is partly contracted, which it always is when the external light is increased, then the illumination is more obfcure, and appears to come from the iris; a part of the light reflected from the tapetum being thrown back, by the concave furface of the cornea, upon the anterior surface of the iris, giving it a bright fhining appearance. "The influence which the will of the animal has over this luminous appearance, feems altogether to depend on the contraction and relaxation of the iris. When the animal is alarmed, or firft difturbed, it naturally dilates the pupil, and the eye glares; when it is appeafed or compofed, the pupil contracts, and the light in the eye is no longer feen. "The most material information that has been gained in this inveftigation, is the tranfparent state of the retina in the eye during life; the opaque membranous appearance, which it puts on in the dead body, not being natural to it, but a change which takes place in confequence of of death. This fact is almoft all that is neceffary to explain the luminous appearance in the eyes of cats." P. 3. Having found the retina to be perfectly tranfparent, when the eye is examined in a very recent ftate, Mr. H. was led to fufpect, that the internal part of the optic nerve, when examined in the fame ftate, might alfo be tranfparent; and, for the purpofe of determining this matter, made the following experiment: "The pofterior half of a cat's eye, while in a very recent state, was immerfed in a bason of water, and examined. The tapetum ap peared very bright, the retina not having acquired fufficient opacity to become vifible. The entrance of the optic nerve was a very white fpot, which feemed to be opaque; but, when small pieces of coloured paper were alternately placed between the outfide of the eye and the bottom of the bafon, their colour was diftinétly feen in the cavity of the eye, through the fubftance of the optic nerve; fo that, at this part, the internal structure of the nerve has a degree of transparency." P. 6. After having afcertained the femi-tranfparent nature of the internal parts of the recent optic nerve, Mr. H. was induced to examine its fubftance, by means of magnifying glaffes. But, previous to the account of his own experiments, he gives a fhort view of the principal theories concerning the structure of nerves, that have been advanced by other philofophers, namely, Torre, Prochaska, Fontana, and Dr. Monro. The microscope ufed by Mr. H. in thefe experiments, was a fingle lens, that magnified the object about twenty-three times; and the objects were attentively viewed both by himfelf and by Mr. Ramfden. The experiments were made, with all due care and attention, both on transverse and on longitudinal fections of the optive nerve of the horse, recently dead. The refult is as follows: "At its origin from the brain, it confifts of 30 or 40 fafciculi or bundles of extremely fmall opaque pulpy fibres, the interftices between which are filled with tranfparent jelly. As the nerve goes farther from the brain, the fafciculi form fmaller ones, of different fizes. This is not done by a regular fubdivifion, but by a few fibres going off laterally from feveral large fafciculi, and being united, forming a fmaller one. Some of the fafciculi fo formed, which are very small, unite again into one. In this way the fafciculi gradually diminish' in fize, and increase in number, till they terminate in the retina. "Near the eye, where the fafciculi are moft numerous, the fubftance of the nerve has a confiderable degree of tranfparency, from the number of tranfparent interftices between them; but this is lefs the cafe nearer the brain, where the interftices are fewer. "In the optic nerve of the cat, the ftructure is the fame as in the horfe; but, from the fmallness of the parts, lefs fitted for investigation. A 2 Near Near the eye, its internal fubftance is more tranfparent than the correfponding part in the horse. To fee how far this ftructure was peculiar to the optic nerve, fimilar experiments were made upon the internal fubftance of the fifth and feventh pair of nerves, near the origin at the brain, and the ftructure was found to be the fame. In thefe laft-mentioned nerves, the interstices between the fafciculi were fmaller than in the optic nerve, rendering their tranfverfe fections lefs tranfparent; from which, it is natural to fuppofe that the internal parts of the optic nerve are not fo compact as in other nerves, and therefore it is better fitted for examination. "Thefe experiments fhow, that the nerves do not confist of tubes conveying a fluid, but of fibres of a peculiar kind, different from every thing elfe in the body, with which we are acquainted. The courfe of these fibres is very curious; they appear to be conftantly paffing from one fafciculus to another, fo as to connect all the different fafciculi together by a mixture of fibres. This is different from the courfe of blood veffels, lymphatics, or muscular fibres: the only thing fimilar to it, is in the formation of nervous plexufes; which leads to the idea of its anfwering an effential purpose, refpecting the functions of the nerves." P. 11. II. The Bakerian Lecture. Obfervations upon an unusual horizontal Refraction of the Air; with Remarks on the Variations to which the lower Parts of the Atmosphere are fometimes Subject. By the Rev. S. Vince, A. M. F. R. S. and Plumian Profeffor of Aftronomy and Experimental Philosophy in the University of Cambridge. P. 13. The author of this paper defcribes, and endeavours to explain, a fingular effect of horizontal refraction, obferved by himfelf at Ramfgate, on the 1st of Auguft laft. The obfervations were made through a telescope that magnified between 30 and 40 times; though the phænomenon was vifible to the naked eye. When the mafts of a ship were juft feen above the horizon of the fea, there appeared at the fame time in the field of view of the telescope, two complete images of the fhip in the air, vertical to the ship itself. One of thofe images was inverted, and the other erect, having their hulks joined. Similar double images were also seen of other objects; but we are unable to give our readers a more adequate idea of this unusual phænomenon, as also of its explanation, without the afliftance of the plate which accompanies the paper itself. III. Abflract of a Regifter of the Barometer, Thermometer, and Rain, at Lyndon, in Rutland, 1797. With fome Remarks on the Recovery of injured Trees. By Thomas Barker, Efq. P.24. This This Register gives the highest, the lowest, and the mean height of the barometer, and of the thermometer, both within and without the houfe; together with the quantity of rain, for every month throughout the year 1797. The rain, upon the whole, amounts to 27,854 inches. This Regifter, which occupies only one page, is followed by fome general remarks on the peculiar nature of the seasons, the produce of the ground, &c. With refpect to the recovery of injured trees, Mr. Barker relates two cafes of young oak-trees, that having been broken by accident, the upper parts were cut off; but, as the part that had remained in the ground put out fhoots, he trained up the best shoot of each tree, which, after a certain time, appeared likely to become good trees. IV. Some Additions to a Paper, read in 1790, on the Subject of a Child with a double Head. By Everard Home, Efq. F. R. S. P. 28, In the 80th volume of the Philofophical Tranfactions there is a paper of the fame author, giving an account of a child with a double head. The child had been born in India, where he died when he was upwards of four years old; his double fkuli was fent to England, and was exhibited at a meeting of the Royal Society. Mr. Dent, the gentleman who had tranfmitted the above-mentioned fkull from India, returning not long ago to England, furnished Mr. Home with further particulars relative to the double-headed child, and likewife with two portraits of him; namely, a front view and a profile of the double head, taken whilft the child was living, by a Mr. Devis, an artift of confiderable merit. Two copper-plate engravings of the portraits are annexed to the paper. The additional remarks, which were furnished by Mr. Dent, are as follows: The child was a male. The mother had had three other children naturally formed. She could affign no imaginary or accidental caufe during her pregnancy, that might have been productive of the monstrous child. The body of the child was uncommonly thin. "The neck of the fuperior head was about four inches long; and the upper part of it terminated in a hard, round, griftly tumour, nearly 4 inches in diameter, The front teeth had cut the gums in the upper and lower jaw of both heads. "When the child cried, the features of the fuperior head were not always affected; and when it fmiled, the features of the fuperior head did not fympathize in that action, "The |