、

XI. On the MOTION of LIGHT, as affected by refracting and

reflecting Subftances, which are alfo in Motion. By JOHN ROBISON, M. A. F. R. S. EDIN. and Profeffor of Natural Philofophy in the University of Edinburgh.

[Read by Mr PLAYFAIR, April 7. 1788.]

EW of the mathematicians and philofophers of the prefent age have acquired a greater or better founded reputation than the celebrated Abbé BoscovICH; and there is none from whose writings I have received fuch variety of inftruction and entertainment. His Theory of Natural Philofophy will ever be confidered by impartial judges, not only as one of the boldest, but also as one of the most ingenious researches into the fecrets of nature. There is hardly a branch of phyficomathematical philosophy which he has not cultivated with fuccefs; and in this cultivation he has exhibited the most acute penetration and the greatest addrefs. In all his investigations too he has given the most beautiful fpecimens of geometrical invention and elegance, and greatly heightens the pleasure of his readers, by marking out diftinctly the progrefs of his own mind in his researches.

ed

Mr BOSCOVICH has lately obliged the public with a collection of feveral of his smaller works in five volumes quarto, publishat Baffano in 1785. In the fecond and fourth volumes of this collection, are two very curious papers, on what is called the aberration of light, or the effect which is produced on the apparent place of visible objects by the motion of the observer.

There is one deduction which he makes from his premises, extremely curious in itself, and having the most surprising confequences. It is this: If a telescope be conftructed, having its tube filled with water, and be directed to a terreftrial object properly fituated, it will be found to deviate from that object by a certain determined quantity every day. It will follow from this, that a person shut up in a mine or dungeon, may, without feeing the fun or heavens, difcover the motion of the earth round the centre of the folar fyftem, and also whether this centre be in motion, and the velocity and direction of this motion.

THE contrivance of a telescope filled with water, has been long familiar to my thoughts, (as a means of discovering whether light be accelerated when refracted towards the perpendicular) in confequence of the fpeculations of my ingenious friend Profeffor WILSON of Glasgow. But all my attempts to conftruct fuch a telescope have hitherto proved abortive, for want of a fubftance fufficiently transparent to admit of the necessary magnifying power. I faw that this rendered useless the beautiful theory of their construction which is contained in this paper of Mr BoscovICH. But, at the fame time, I faw that this aberration of terrestrial objects would enable us to decide the fame question by means of a compound microscope of a very easy conftruction. If a cylindrical piece of glass be ground spherical at one end, and plane at the other, and if the plane furface be fituated a finall distance beyond the principal focus of the spherical surface, and a scratch be made on the plane furface, and confidered as a visible object, an image of this scratch will be formed in the conjugate focus of the spherical furface, which image may be viewed by means of a deep eye-glass, as in the ordinary compound microscope. If this image be formed on a frame of wires, like the wires of an astronomical telescope, there must be obferved the fame diurnal deviation that Mr BoscovicH announces with respect to his tele

fcope,

fcope, but in the oppofite direction; as in the microfcope, there would be no want of light, we should have the most fatisfactory decifion of this important queftion in optics, and alfo the opportunity of detecting any hitherto unknown motions of the globe which we inhabit. It may also be shown, that, if any of these motions be very confiderable, we fhall determine another very important question in optics, viz. Whether the motion of light be affected by the motion of the luminous body.

ON these, and many other accounts, I was eager to construct this microscope, and set about it accordingly. But I happened at that time to be engaged in that part of my courfe of lectures where I had occafion to confider the apparent motions of bodies. I confider it as the fundamental propofition on this fubject, that "the apparent motion of a body is compounded of its real

motion, and the oppofite to the real motion of the obferver." The confequence is, that, fince the motions of the terrestrial object and of the observer are always nearly equal, there should be no apparent motion in the object, and therefore no apparent diurnal change of place. This startled me, and caused me to confider the matter more minutely. Profeffor WILSON, to whom I communicated my doubts, raised other objections, founded on the application of mechanical principles to that hypothesis, with refpect to light, which the Abbé BoscovICH profeffes to maintain. In my fubfequent speculations on this subject, I found, that the application of the above mentioned propofition was not strictly just with respect to the apparent place of the terrestrial object; but I was led by it to discover the real state of the matter, by applying it to the determination of the apparent motion of the light by which the object is feen. I thus detected the circumftance which Mr BoscoVICH had overlooked, and which unfortunately puts an end to the hopes which I had entertained of many curious and important discoveries. I flatter myself that this Society will not think this fubject unworthy of their notice; but am extremely forry that my infirm ftate of

health

health does not at prefent permit me to give fuch an account of it as its importance deferves. I propose, however, to undertake it as foon as I am able. This I am incited to do, not merely on account of the fingularity of this particular subject, but more especially because its difcuffion depends on a more general, and hitherto unconfidered subject in phyfico-mathematical science, the motion of light as affected by bodies which are also in motion. This I have confidered fome years ago, as far as I thought neceffary for my elementary courfe of lectures, and I then inveftigated the fundamental propofition which I fhall include in this discourse. Perhaps I fhould offer fome apology for troubling the Society with my thoughts on the subject before I have put them into a more perfect form. I fhall frankly tell I shall frankly tell my reasons for this conduct. This paper of Mr BoscovICH muft excite the attention of philofophers. Other speculations also which have lately been made by ingenious men, will turn the attention to the fubject, and enquiries will be inftituted, and their results made public. I should not chuse to be thought indebted to the refearches of others for the refults of my own enquiries, and therefore wifh to afcertain my claim to any thing which may be valuable in my speculations, by this present imperfect account of them.

I SHALL therefore lay before the Society a fhort account of the experiment, as proposed and described by Mr BOSCOVICH, and of the refult which he expects from it, and fome of the most remarkable confequences which he deduces from this refult. I fhall, in the next place, point out the overfight which he has made in announcing the refult, and ftate what ought to be the refult, on the physical principles adopted by him; principles which will be overturned if the refult of the experiment should be what he expects, but established if it should be what I affert. In the last place, I fhall give the fundamental propofition for determining the reflection and refraction of light

by