The object of this paper is certainly of great importance; and every attempt to effect it is very meritorious. We are happy to find that, in the conftruction of a new inftrument, Mr. W. has fucceeded fo much to his own fatisfaction; and we hope that his fuccefs will induce others to direct their attention to this fubject, and to make thofe farther improvements in this useful inftrument of which it is capable.

Obfervations on the Planet Venus. By William Herfchel, LL.D.

F. R. S.

Dr. Herschel has purfued, from April 1777 to the prefent time, a series of obfervations on the planet Venus. His firft view, in these obfervations, was to afcertain the diurnal rotation of this planet; the duration of which, after the contradictory accounts of Caffini and Bianchini, (one of whom flates it at 23 hours and the latter at 24 days,) remains unknown. Another object was the atmosphere of this planet, of the exiftence of which he does not entertain the leaft doubt. investigation of its real diameter was the third object; and the construction of the planet, with regard to fuch permanent appearances as might be occafioned by feas, continents, or mountains, was a fourth object of his attention.

The

The refult of his obfervations would long ago have been. communicated to the public, if he had not waited in expectation of being able to determine the period and direction of the diurnal motion of Venus; and he should even now have hefitated to transcribe the extracts of his journal contained in this paper, if it did not feem incumbent on him to examine by what accident he had overlooked mountains in this planet, which are faid to be of fuch enormous height, as to exceed four, five, and even fix times the perpendicular elevation of Chimboraco, the highest of our mountains:' i. e. affuming the highest number, 23 miles. Thefe and other phenomena, equally wonderful, recited by Mr. Schroeter in the Phil. Tranf. for 1792, Part II. (See Rev. N. S. vol. ii. p. 85.) have efcaped the notice of Dr. Herschel; and he is now induced to communicate his obfervations, in order to convince the public that neither want of attention nor a deficiency of inftruments could occafion his not perceiving these mountains of more than 23 miles in height, the jagged border of Venus, and the flat spherical forms on Saturn, which Mr. S. imagined that he had dif covered.

Dr. H. after having recited a variety of obfervations, the accuracy of which none will be difpofed to queftion, deduces from them the following general inferences. With regard to the rotation of Venus on its axis, he concludes that, though it has undoubt

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edly a diurnal motion, the time of it is uncertain. He apprehends, however, that it cannot be fo flow as 24 days. The polition of its axis is ftill more uncertain. The atmosphere of Venus is probably very confiderable; as he infers from the changes that have been obferved in the faint fpots on its furface, and alfo from the illumination of the cufps, when this planet is near its inferior conjun&tion; in which cafe, the enlightened ends of the horns reach far beyond a femicircle. Dr. H. acknowleges that the meafures, on which he founded his calculations of the extent of the twilight of Venus, were neither fo numerous nor fo fatisfactory as he wifhed: but his computations, compared with thofe that occur in the paper to which we have referred, fhew with fufficient evidence that it is of much greater extent, or refractive power, than the writer of it has allowed.

As to the mountains of Venus, no eye, he fays, which is not confiderably better than his, or affifted by much better inftruments, will ever get a fight of them. Arguing, however, from the analogy that obtains between the only two planetary globes, which we can compare, viz. the moon and the earth, he thinks that there is little doubt that this planet has inequalities on its furface, which may be very confiderable. The apparent diameter of Venus, at the mean diftance from the earth, he states to be 18",79. Hence we may infer that this planet is fomewhat larger than the earth, inflead of being lefs, as aftronomers have imagined. On the niceft fcrutiny, he cannot find fault with the measures whence this conclufion was drawn but these measures fhould be repeated, in order to our being able to fpeak confidently on this fubject. Dr. H. had many opportunities of obferving a luminous margin round the limb of Venus:

With regard to the caufe of this appearance, (fays the Doctor,) I believe that I may venture to afcribe it to the atmosphere of Venus, which, like our own, is probably replete with matter that reflects and refracts light copiously in all directions. Therefore on the border, where we have an oblique view of it, there will of confequence be an increase of this luminous appearance. I fuppofe the bright belts and polar regions of Jupiter, for infance, which have a greater light than the faint freaks, or yellow belts, on that planet, to be the parts where its atmosphere is most filled with clouds; while the latter are probably thofe regions which are free from them, and admit the fun to fhine on the planct; by which means we have the reflection of the real furface, which I take to be generally lefs luminous. If this conjecture be well founded, we fee the reafon, why spots on Venus are fo feldom to be perceived. For this planet having a denfe atmosphere, its real furface will commonly be enveloped by it, fo as not to prefent us with any variety of appearances. This alfo points

out the reafon, why the fpots, when any fuch there are, appear generally of a darker colour than the rest of the body.'

PHILOSOPHICAL PAPERS, &c.

Defcription of an Inftrument for afcertaining the specific Gravities of Fluids. By John Godfrey Schmeiffer.

This inftrument confits of a flat-bottomed glafs veffel, to which is adapted a glafs topper with a thermometer palling through it; the bore of the ftopper is conical; and the thermometer has a glafs collar, which is ground into the bore of the ftopper, fo as to be perfectly tight. When the tube of the thermometer and the collar are not of the fame metal, a very thin piece of elaftic gum may be wound about the tube, and by this method it may be fixed in the ftopper. This gum will exclude air and liquids; and, in the ufual temperature of the atmofphere, no liquor will diffolve it, except vitriolic æther particularly prepared for the purpose. The cavity at the upper part of the ftopper, which does not come in contact with the liquors that are to be weighed, may be filled up with fealingwax, or any other cement. The feveral parts of this inftrument are exhibited in a drawing annexed to this paper; and the inventor has particularly defcribed the method of ufing it. He contrives to adjust the temperature of the various Auids, the fpecific gravities of which are to be determined, to the common ftandard of 60°, by fetting the bottle, containing the fluid, in a glafs veffel with cold water, and adding as much warm water as is neceffary to bring the fluid to this ftandard.

Extract of a Letter from Sir Charles Blagden, Sec. R. S. giving fame Account of the Tides at Naples.

The rife and fall of the tides at Naples are fo inconfiderable, that it is difficult to afcertain them. From the beft obfervation which Sir C. Blagden had an opportunity of making, as well as from fome others lefs accurate, he infers that the time of high water at full and change in the bay of Naples is between nine and ten o'clock A. M.

Obfervations on Vision. By Thomas Young.

Experience teaches us that the eye is capable of viewing objects at a certain diftance, without any mental exertion. Beyond this distance, no mental exertion can be of any avail: but, within it, the eye poffeffes a power of adapting itself to the various occafions that occur, the exercife of which depends on the volition of the mind. How this is effected, is a problem. that has very much engaged the attention of optical writers: but it has not yet been fatisfactorily explained. The first theory for the folution of this problem is that of Kepler. He fuppofes

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that the ciliary proceffes contract the diameter of the eye, and lengthen its axis by a mufcular power: but the prefent ingenious author observes that thefe proceffes neither appear to contain any muscular fibres, nor have any attachment by which they can be capable of performing this action. Defcartes afcribed this contraction and elongation to a mufcularity of the crystalline, of which he fuppofed the ciliary proceffes to be the tendons: but he neither demonftrated this mufcularity, nor fufficiently confidered the connection with the ciliary proceffes. De la Hire allows of no change in the eye, except the contraction and dilatation of the pupil. This opinion he founds on an experiment which Dr. Smith has fhewn to be fallacious. Haller adopted his hypothefis, notwithstanding its inconfiftency with the principles of optics and conftant experience. Dr. Pemberton fuppofes that the cryftalline contains mufcular fibres, by which one of its furfaces is flattened, while the other is made convex: but he has not demonftrated the existence of these fibres; and Dr. Jurin has proved that fuch a change as this is inadequate to the effect. Dr Porterfield conceives that the ciliary procefies draw the cryftalline forward, and make the cornea more convex: but the ciliary proceffès are incapable of this action; and it appears, from Dr. Jurin's calculations, that a fufficient motion of this kind requires a very visible increase in the length of the axis of the eye; yet this increase is not obferved. Dr. Jurin maintains that the uvea, at its attachment to the cornea, is mufcular; and the contraction of this ring makes the cornea more convex: but this hypothefis is not fufficiently confirmed by obfervation. Mufchenbroek conjectures that the relaxation of his ciliary zone, which is nothing but the capfule of the vitreous humour where it receives the impreffion of the ciliary procefles, permits the coats of the eye to push forward the cryftalline and cornea. Such a voluntary reJaxation, our author obferves, is wholly without example in the animal economy; and, if it actually occurred, the coats of the eye could not act as he conceives; nor could they act in this manner without being obferved. He adds that the contraction of the ciliary zone is equally inadequate and unneceffary.

Mr. Young, having examined these theories, and fome others of lefs moment, and fuggefted the objections to which they are liable, proceeds to inveftigate a more probable folution of this optical difficulty.

Adverting to the obfervation of Dr. Porterfield that those who have been couched have not the power of accommodating the eye to different diftances; and to the reflections of other writers on this fubject; he was led to conclude that the rays of light, emitted by objects at a fmall diftance, could only be brought to

foci on the retina by a nearer approach of the crystalline to a fpherical form; and he imagined that no other power was capable of producing this change, befide a mufcularity of part or of the whole of its capfule:-but, on closely examining, firft with the naked eye and then with a magnifier, the crystalline of an ox's eye turned out of its capfule, he discovered a structure which feemed to remove the difficulties that have long embarraffed this branch of optics.

The cryftalline of the ox is compofed of various fimilar coats, each of which confifts of fix inches, intermixed with a gelatinous fubftance, and attached to fix membranous tendons. Three of the tendons are anterior, three pofterior; their length is about twothirds of the femi-diameter of the coat; their arrangement is that of three equal and equidiftant rays, meeting in the axis of the cryftalline: one of the anterior is directed towards the outer angle of the eye, and one of the pofterior towards the inner angle, fo that the pofterior are placed oppofite to the middle of the interftices of the anterior: and planes paffing through each of the fix, and through the axis, would mark on either furface fix regular equidistant rays. The mufcular fibres arife from both fides of each tendon; they diverge till they reach the greateft circumference of the coat; and, having paired it, they again converge, till they are attached refpectively to the fides of the nearest tendons of the oppofite furface. The anterior or pofterior portion of the fix, viewed together, exhibits the appearance of three penniform-radiated mufcles. The anterior tendons of all the coats are fituated in the fame planes, and the pofterior ones in the continuations of thefe planes beyond the axis. Such an arrangement of fibres can be accounted for on no other fuppofition than that of mufcularity. This mafs is inclosed in a strong membranous capfule, to which it is loosely connected by minute veffels and nerves; and the connection is more obfervable near its greatest circumference. Between the mass and its capfule is found a confiderable quantity of an aqueous fluid, the liquid of the cryftalline.

When the will is exerted to view an object at a small distance, the influence of the mind is conveyed through the lenticular ganglion, formed from branches of the third and fifth pairs of nerves, by the filaments perforating the fclerotica, to the orbiculus ciliaris, which may be confidered as an annular plexus of nerves and veffels; and thence by the ciliary proceffes to the mufcle of the crystalline, which, by the contraction of its fibres, becomes more convex, and collects the diverging rays to a focus on the retina. The difpofition of fibres in each coat is admirably adapted to produce this change; for, fince the least surface that can contain a given bulk is that of a fphere (SIMPSON'S Fluxions, p. 486.) the contraction of any furface muft bring its contents nearer to a spherical form. The liquid of the cryftalline feems to ferve as a fynovia in facilitating the motion, and to admit a fufficient change of the mufcular part, with a fmaller motion of the capfule.'

The author proceeds to inquire whether thefe fibres can produce an alteration in the form of the lens fufficiently great to account

for