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the light of Sirius and 3 Tauri, a ftar of the fecond magnitude, is not more.than as 4 to 1; while that between the for mer and the fun is as 170,000 millions to 1. The next difference between stars of the fecond and third magnitude is only as 2 to 1; that between stars of the fixth and feventh magnitude only as 14 to 1. With the naked eye, and with objects of no greater brightness than stars, we cannot probably penctrate farther into space; but clusters of stars, which form nebulæ, are feen at a ftill greater diftance.

Our author next examines what further affistance telescopes can be expected to give. Some light must be loft by paffing through the glaffes. In his beft telescope, not above .63 of the rays reach the eye: in a Newtonian reflector, with a double eye-glafs, not many more than 40. On examination, with one of our author's twenty-feet Newtonian reflectors, made in 1776, he found that, with its affiftance, he could penetrate thirty-nine times farther into space than with the naked eye. In this cafe, the abfolute not the intrinfic brightnefs is increased. This leads Mr. Herschel to the diftinction between magnifying and penetrating power, the latter of which only is poffeffed by the night-glaffes, which penetrate fix or feven times farther than the natural eye; and the great advantages of our author's telescopes arife from their combining the penetrating and magnifying power. Various inftances of the utility of occafionally increafing the one or other of thefe powers are fubjoined, which can only be read with advantage in his own words. In fome circumftances, however, thefe powers interfere with each other; and even the magnifying power has its maximum, fince, by extending it too far, obfcurity enfues from magnifying the medium. In fome nights, when the air is full of vapour, but not in the velicular ftate, there are fcarcely any limits to the magnifying power. The penetrating power may alfo, in our author's opinion, be greatly extended. His forty-feet reflector advances to 191.69, but he thinks it poffible to extend this power fo far as 500. Even with his reflector, allowing a star of the feventh magnitude to be vifible to the unaffifted eye, this telefcope will thow ftars of the 1342d magnitude; but, when affifted by the united luftre of fidereal fyftems, it will penetrate 11 millions of millions of millions of miles, exceeding 300,000 times the distance of the nearcft fixed ftar! The range of fuch a telefçope must be of courfe extenfive beyond imagination, and to examine these immense diftances there are few favourable hours. Mr. Herfchel, from his journal, thinks that a year, which affords go or 100 of thefe hours, is very productive; and to fweep the heavens' with his twenty-feet reflector, would require 14 of fuch productive years; and, with the forty-feet reActor, with the power of 1000, it will require 598 of fuch

years, leaving fo much of the fouthern hemifphere as will require 213 years more, allowing only one fingle moment to look into each part of space.

V. A fecond Appendix to the improved Solution of a Problem in phyfical Aftronomy, inferted in the Philofophical Tranfactions for the Year 1798, containing fome further Remarks, and improved Formule for computing the Coefficients A and B; by which the arithmetical Work is confiderably fhortened and facilitated. By the Rev. John Hellins, B. D. F. R. S. and Vicar of Potter's Pury, in Northamptonshire.' This excellent paper can only be examined with advantage in the volume.

VI. Account of a Peculiarity in the Diftribution of the Arteries fent to the Limbs of flow-moving Animals; together with fome other fimilar Facts. In a Letter from Mr. Anthony Carlife, Surgeon, to John Symmons, Efq. F. R. S.'

The diftribution of the blood-veffels, except in the fuperior and inferior extremities, offers nothing very ftriking; but, in thefe, the artery is divided at once into very many cylindrical branches, which often anaftomofe. The final caufe of this fingular arrangement is not clear. Our author thinks it is connected with the power the animal has of keeping itfelf, for a long time, fufpended; in other words, that it affifts the muf cles in preferving their permanent contraction, without alternate relaxation. It feems more probably defigned to prevent obstructions, in confequence of the continued action of the muscles, or their flow motion; for, in the more active bradypus, the B. tridactylus, the divifion is much less minute.

VII. Outlines of Experiments and Inquiries refpecting Sound and Light. By Thomas Young, M. D. F. R. S. In a Letter to Edward Whitaker Gray, M. D. Sec. R. S.'

As the completion of Dr. Young's purfuits on this fubject is yet at a distance, he has here publifhed fome of his conclufions, left, from accident, he may not be able to continue the inquiry. The fubjects are,

I. The measurement of the quantity of air difcharged through an aperture. II. The determination of the direction and velocity of a stream of air proceeding from an orifice. III. Ocular evidence of the nature of found. IV. The velocity of found. V. Sonorous cavities. VI. The degree of divergence of found. VII. The decay of found. VIII. The harmonic founds of pipes. IX. The vibrations of different elaftic fluids. X. The analogy between light and found. XI. The coalefcence of mufical founds. XII. The frequency of vibrations conftituting a given note. XIII. The vibrations of chords. XIV. The vibrations of rods and plates. XV. The human voice. XVI. The temperament of musical intervals. P. 106.

It is impoffible to follow minutely experiments reduced to the form of tables, and difquifitions, which contain a large portion of mathematical reafoning, and frequent reference to plates. We fhall, therefore, only notice a few of the most ftriking or important paffages, which do not require fuch affiftance.

On the fubject of fonorous cavities, our author confirms the obfervation of de la Grange, that founds are reflected with a velocity equal to that of their impulfe. When the walls of an unfurnished narrow room are parallel and smooth, found is reflected from one to the other fide, and it takes place, as frequently in a second, as double the breadth of the room is contained in 1130 feet. The appropriate notes of a room may be afcertained by finging the fcale in it, and will be found to depend on the proportion of its length or breadth, to 1130 feet.

He oppofes the idea of the divergence of found, with great juftice. It is only surprising that this opinion has prevailed fo long. Sound, he thinks, decays in the duplicate ratio of the diftance, and, of courfe, the propofal of the improved form of the fpeaking trumpet, to reprefent the logarithmic curve, is fallacious. In the tenth fection, on the analogy between light and found, Dr. Young offers fome remarks in favour of EuJer's fyftem of light being propagated by an etherial medium.

There are also one or two difficulties in the Newtonian system, which have been little obferved. The firft is, the uniform velocity with which light is fuppofed to be projected from all luminous bodies, in confequence of heat, or otherwife. How happens it that, whether the projecting force is the flighteft tranfmiffion of electricity, the friction of two pebbles, the loweft degree of visible ignition, the white heat of a wind furnace, or the intenfe heat of the fun itself, these wonderful corpufcles are always propelled with one uniform velocity? For, if they differed in velocity, that difference ought to produce a different refraction. But a still more infuperable difficulty feems to occur, in the partial reflection from every refracting furface. Why, of the fame kind of rays, in every circumftance precifely fimilar, fome fhould always be reflected, and others tranfmitted, appears in this fyftem to be wholly inexplicable. That a medium resembling, in many properties, that which has been denominated ether, does really exift, is undeniably proved by the phænomena of electricity; and the arguments against the existence of fuch an ether throughout the univerfe, have been pretty fufficiently answered by Euler. The rapid tranfmiflion of the electrical fhock fhows that the electrical medium is poffeffed of an elafticity as great as is neceffary to be fuppofed for the propagation of light. Whether the electric ether is to be confidered as the fame with the luminous ether, if fuch a fluid exifts, may perhaps at fome future time be difcovered by experiment; hitherto I have not been able to

obferve that the refractive power of a fluid undergoes any change by electricity. The uniformity of the motion of light in the fame medium, which is a difficulty in the Newtonian theory, favours the admiffion of the Huygenian; as all impreffions are known to be tranfmitted through an elastic fluid with the fame velocity. It has been already fhown, that found, in all probability, has very little tendency to diverge: in a medium fo highly elaftic as the luminous ether must be fuppofed to be, the tendency to diverge may be confidered as infinitely fmall, and the grand objection to the fyftem of vibration will be removed. It is not abfolutely certain, that the white line vifible in all directions on the edge of a knife, in the experiments of Newton and of Mr. Jordan, was not partly occafioned by the tendency of light to diverge. Euler's hypothefis, of the tranfmiffion of light by an agitation of the particles of the refracting media themselves, is liable to ftrong objections; according to this supposition, the refraction of the rays of light, on entering the atmosphere from the pure ether which he describes, ought to be a million times greater than it is. For explaining the phænomena of partial and total reflection, refraction, and inflection, nothing more is neceffary than to fuppofe all refracting media to retain, by their attraction, a greater or lefs quantity of the luminous ether, fo as to make its denfity greater than that which it poffeffes in a vacuum, without increafing its elafticity; and that light is a propagation of an impulfe communicated to this ether by luminous bodies: whether this impulfe is produced by a partial emanation of the ether, or by vibrations of the particles of the body, and whether these vibrations are, as Euler fuppofed, of various and irregular magnitudes, or whether they are uniform, and comparatively large, remains to be hereafter determined. Now, as the direction of an impulfe tranfmitted through a fluid, depends on that of the particles in fynchronous motion, to which it is always perpendicular, whatever alters the direction of the pulfe, will inflect the ray of light. If a smaller elaftic body ftrike against a larger one, it is well known that the finaller is reflected more or lefs powerfully, according to the difference of their magnitudes: thus, there is always a reflection when the rays of light pafs from a rarer to a denfer ftratum of ether; and frequently an echo when a found ftrikes against a cloud. A greater body ftriking a fmaller one, propels it, without lofing all its motion: thus, the particles of a denfer ftratum of ether do not impart the whole of their motion to a rarer, but, in their effort to proceed, they are recalled by the attraction of the refracting fubftance with equal force; and thus a reflection is always fecondarily produced, when the rays of light pafs from a denfer to a rarer ftratum,' P. 125.

It has already been conjectured by Euler, that the colours of light confift in the different frequency of the vibrations of the luninous ether it does not appear that he has fupported this opinion

by any argument; but it is ftrongly confirmed, by the analogy be tween the colours of a thin plate and the founds of a feries of or gan-pipes. The phænomena of the colours of thin plates require, in the Newtonian fyftem, a very complicated fuppofition, of aa ether, anticipating by its motion the velocity of the corpufcles of light, and thus producing the fits of tranfmiffion and reflection; and even this fuppofition does not much aflift the explanation. It appears, from the accurate analysis of the phænomena which Newton has given, and which has by no means been fuperfeded by any later obfervations, that the fame colour recurs whenever the thicknefs answers to the terms of an arithmetical progreffion. Now this is precifely úmilar to the production of the fame found, by means of an uniform blaft, from organ-pipes which are different multiples of the fame length. Suppofing white light to be a continued impulfe or ftream of luminous ether, it may be conceived to act ou the plates as a blaft of air does on the organ-pipes, and to produce vibrations regulated in frequency by the length of the lines which are terminated by the two refracting furfaces. It may be objected that, to complete the analogy, there fhould be tubes, to answer to the organ-pipes but the tube of an organ-pipe is only neceffary to prevent the divergence of the impreffion, and in light there is little or no tendency to diverge; and indeed, in the cafe of a refonant paffage, the air is not prevented from becoming fonorous by the liberty of lateral motion. It would feem, that the determination of a portion of the track of a ray of light through any homoge neous ftratum of ether, is fufficient to establish a length as a bafis for colorific vibrations. In inflections, the length of the track of a ray of light through the inflecting atmosphere may determine its vibrations: but, in this cafe, as it is probable that there is a reflection from every part of the furface of the furrounding atmosphere, contributing to the appearance of the white line in every direction, in the experiments already mentioned, fo it is poffible that there may be fome fecond reflection at the immediate furface of the body itfelf, and that, by mutual reflections between thefe two furfaces, fomething like the anguiform motion fufpected by Newton may really take place; and then the analogy to the colours of thin plates will be ftill ftronger. A mixture of vibrations, of all poffible frequencies, may easily destroy the peculiar nature of each, and concur in a general effect of white light.' P. 128.

On this fubject we can offer no remarks, as they would lead us to confiderable and difproportioned digreffions. We may, however, obferve, that the advantages of Euler's hypothefis, thus detailed, are partial only, and refer but to one point of the fubject. The difadvantages and the difcordance of this fyftem to numerous facts, will be very obvious to the experienced philofopher; but they appear to us to merit investigation in the

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