On the Affections and Properties of Light. 151 ing body's action reaches farther when exerted on some rays than when exerted on others : but then, the direction of the rays not pafling through the body, thofe which are fartheft off and at too great a distance to be bent, never coming nearer, are not bent at all; and confequently as the least flexible rays are in this predicament at the fmalleft diftance, and the moft flexible not till the diftance is greater, the images formed 'out of the former must be less than thofe formed out of the latter. This difference in the way in which the phenomenon appears, does not argue the smallest difference in the cause it only follows from the different pofition of the rays, with respect to the acting body, in the two cafes. I infer then from the whole, that different forts of rays come within the fpheres of flexion, reflexion, and refraction, at different diftances, and that the actions of bodies extend fartheft when exerted on the most flexible. It may perhaps be confiftent with accuracy and convenience to give a name to this property of light; we may therefore fay that the rays of light differ in degree of refrangity, reflexity, and flexity, comprehending inflexity and deflexity. From these terms (uncouth as, like all new words, they at first appear) no confufion can arife, if we always remember that they allude to the degree of diftance to which the rays are fubject to the action of bodies. I fhall only add an illustration of this property, which may tend to convey a clearer idea of its nature. Suppofe a magnet to be placed so that it may attract from their course a stream of iron particles, and let this ftream pass at fuch a diftance that part of it may not be affected at all: thofe particles which are attracted may be conceived to strike on a white body placed beyond the magnet, and to make a mark there of a fize proportional to their number. Let now another equal ftream confiderably adulterated by carbonaceous matter, oxygen, &c. pafs by at the fame diftance, and in the fame direction. Part of this will also be attracted, but not fo far from its courfe, nor will an equal number be affected at all; fo that the mark made on the white body will be nearer the direction of the ftream, and of less fize than that made by the pure iron. It matters not whether all this would actually happen, even allowing we could place the fubjects in the fituation described: the thing may easily be conceived, and affords a good enough illuftration of what happens in the cafe of light. Pursuant to the plan I before followed, I now tried to measure the different degrees of reflexity, &c. of the different rays; but though the measurements which I took agreed in this, that the red images were much larger than the reft, and the green appeared by them of a middle fize, yet they did not agree well enough (from the roughnefs of the images, and feveral other caufes of error) to authorize us to conclude with any certainty " that the action of bodies on the rays is in proportion to the relative fizes of thefe rays." This, however, will moft probably be afterwards found to be the cafe: in the mean time there is little doubt that the fizes are the cause of the fact. II. SEVERAL phenomena are eafily explicable on the principles juft now laid down. 1. If a pin, hair, thread, &c. be held in the rays of the fun refracted through a prism, extending through all the feven colours, a very fingular deception takes place: the body appears of different fizes, being largest in the red, and decreasing gradually towards the violet, This appearance feemed fo extraordinary, that fome friends who happened to fee it as well as myself, 152 On the Affections and Properties of Light. myself, fufpected the body must be irregular in its shape. On inverting it, however, the fame thing took place; and on turning the prifm on its axis, fo that the different rays fucceffively fell on the fame parts, the visible magnitude of the body varied with the rays that illuminated it. This appearance is readily accounted for by the different reflexity of the rays, and follows immediately from Obfervation 2d and 3d. 2. Sir Ifaac Newton found that the rings of colours made by thin plates, and by thick plates of glass (as he calls them), when formed of homogeneal light, varied in fize with the rays that made them, being largest in the most flexible rays. I have had the pleasure of obferving feveral other forts of rings, fo extremely fimilar, and formed by flexion, that I can no longer doubt of this being alfo the cause of the phenomena obferved by Newton. I fhall 'first defcribe a fpecies, to prove "that the colours by thick and thin plates are one and the fame phenomena, only differing in the thickness of the plates." Happening to look by candle light upon a round concave plate of brafs, pretty well polished, fo as to reflect light enough for fhewing an image of the candle, I was furprised to fee that image furrounded by feveral waves of colours, red, green, and blue, difpofed in pretty regular order. This was fo uncommon in a metallic fpeculum, that I examined the thing very minutely by a variety of experiments: thefe I fhall not particularly now defcribe, but give a general idea of their refults. It must be observed, for the fake of clearnefs, that in the following enquiries concerning the formation of rings or fringes, the diameter of a ring or fringe means the line paffing through the centre of that ring, and terminated at both ends by the circumference; whereas the breadth means that part of the diameter intercepted between the limits of the ring, or the distance between its extreme colours, red and violet. In the first place, they were formed by the fun's light in the figure of rings, furrounding the centre of the sphere to which the plate was ground, at greater diftances increafing their breadths, the colours pretty bright, though inferior in brilliancy to those of concave fpecula. Secondly, the order of the colours was in all red outermoft, and violet or blue innermoft, with a greyish-blue spot in the common centre of the whole; and on moving the plate from the perpendicular position, the rings moved and broke exactly like those of fpecula. In the third place, homogeneal light made them of fimple colours; they were broadeft when red, narroweft when blue and violet. Fourthly, they decreafed in breadth from the centre; and I found by a fimple contrivance, that they were to one another in the very fame ratio that the rays by fpecula follow. In the fifth place, I compared the general appearance of the two forts by viewing them at the fame time, and was ftruck with their general appearance, unless that these of specula were most vivid and distinct. Thefe things made me fufpect that they were actually caufed by the thin coat of gums with which the furface of the plate was varnished, called lacker. Accordingly I took it off with spirit of wine, and found the rings disappear; on lackering it again they returned; and in like manner I caufed a well finished concave metal fpeculum to form the rings of which we are speaking, by giving it a thin coat of lacker. This is a clear proof that thefe Imperfections from Colour in Speculums. 153 rings were exactly the fame with thofe of thick plates (to ufe Newton's expreffion); for the coat of gums is, when thin, pretty tranfparent, as may be seen by laying one on glass plates. But this coat is extremely thin, and cannot exceed the 200th part of an inch; fo that the colours of thick plates are in fact the very fame with thofe of thin plates, except that the two kinds are made by different fized plates. We cannot, therefore, diftinguish them, any more than we do the fpectrum made by a prifm whofe angle is 90° from that made by one whose angle is 20°. This kind of colours is not the only one I have obferved of nearly the fame kind with thofe of plates; we fhall prefently fee another much more curious and re markable. III. IN reflecting on the obfervations and conclufions contained in my former paper, several confequences feemed to follow which appeared fo new and uncommon, that I began to doubt a little the truth of the premises; but at any rate was refolved to examine more minutely how far these inferences might be confiftent with fact: and I am happy in being able to announce the completeness of that confiftency, even beyond my expectations. The chief confequences were the following: 1. That a speculum fhould produce, by flexion and reflexion, colours in its reflected light wherever it has the leaft fcratch or imperfection on its furface. 2. That on great inclinations to the incident rays all fpecula, however pure and highly polished, should produce colours by flexion. 3. That they should also in the fame case produce colours by reflexion. 4. That lenfes, having the smallest imperfections, fhould produce by flexion colours in their refracted light. 5. That there fhould be many more than three, or even four fringes by flexion, invifible to the naked eye. And, 6. That Iceland crystal should have fome peculiarities with refpect to flexion and reflexion; or if not, that fome information should be acquired concerning its fingular properties refpecting refraction. The manner in which the first of these proportions is demonftrated a priori, is evident from the 4th figure, where CD is the reflecting surface, vo a concavity bearing a small ratio to CD, Ao and AB rays proceeding to CD. The one, AB, will be separated into Br red, and Bv violet, by deflexion from o, and will be reflected to r' v ́, forming there the fringes. The other, Ao, being reflected, will be feparated into Bx and By, by deflexion from v, forming other fringes, xy, on the fide of vo's fhadow oppofite to r'v'. Alfo when vo is convex instead of concave, the like fringes will be produced by the rays being deflected in passing by its fides. Lastly, when vo is a polished streak, images by reflexion will be produced, as described Phil. Tranf. for 1796, p. 269. (Philof. J. i. 593.) The fame paffage will alfo fhew the reason why, on great inclinations, colours by reflexion fhould be produced. And the second propofition, with refpect to flexion, follows from what was demonftrated in this paper (p. 149 and 150); it being that cafe where the rays either leave or fall on the speculum at such an inclination as to come only within the sphere of inflexion, without being deflected. The VOL. II-JULY 1798. fourth X 154 Imperfections from Golour in Speculums. fourth propofition is merely a fimple cafe of flexion. And the two laft require no illuftration. I fhall now relate how I inquired into the truth of these things a posteriori. Obfervation 1.-Looking at a plane glafs mirror expofed to the fun's light, I obferved that up and down its furface there were minute fcratches (called hairs by workmen), and that each of these reflected a bright colour, fome red, others green, and others blue. On moving the mirror to a different inclination, or my eye to a different pofition with respect to the mirror, I faw the fpecies of the colours change; the red, for inftance, became green, and the green blue. I applied my eye close to the mirror, and received on it the light reflected from one hair. I obferved feveral diftinct images of the fun much diftended and regularly coloured, just like thofe defcribed above; the fame appearances were obfervable in all fpecula, metal and glafs, which had thefe hairs, and I never faw any metal one without fome: their fize is exceedingly fmall, not above of an inch. Rubbing a minute particle of greafe on the furface of the fpeculum, images were feen on the fibrous furface; and they always lay at right angles to that direction in which the greafe was difpofed by drawing the hand along it. 42 Obfervation 2.-Befides these polished hairs, many specula have fewer or more small specks and threads, rough and black. Perhaps every polished furface is ftudded with a number of fmall ones, invifible to the naked eye from the quantity of regular light which it reflects. I took, from a reflecting telescope, a fmall concave fpeculum not very well finished; its furface fhewed feveral fpecks to the naked eye, and many with a microfcope. Its diameter was of an inch, its focal distance two inches, and the sphere to which it was ground: eight inches diameter. I placed it at right angles to the rays of the fun, coming through a fmall hole of an inch diameter, into a very well darkened room; I then moved it vertically, fo that the rays might be reflected to a chart 12 inches from the fpeculum, and confequently 10 from the focus: and though the focus appeared white and bright, yet on the chart the broad image was very different. It was mottled with a vaft number of dark spots ;. thefe were of two forts chiefly, circular and oblong. Of the former a confiderable number were distinct and large, the reft finaller and more confused, but fo numerous that they feemed to fill the whole image. None were quite black, but rather of a blueifh grey, and the oblong ones had a line of faint light in the middle, juft as is the cafe in fhadows of fmall bodies. But the chief thing which I remarked was the colours. Each oblong and round fpot was bordered by a gleam of white, and several coloured fringes separated by small dark spaces. The fringes were exactly like thofe furrounding the fhadows of bodies, of the fame shape with the dark space, having the colours in the order, red on the outside, blue or violet in the infide-the innermoft fringe was broadeft, the others decreafing in order from the firft. I could fometimes fee four of them, and, when made at the edge of the large image, I could indiftinctly difcern the lineaments of a fifth: when two of the fpots were very near one another, their rings or fringes ran into one another, croffing. Obfervation 3.-When the chart was removed to a greater diftance, as fix feet, the fringes were very diftinct and large in proportion; alfo the smaller spots became more plain, and their rings were feen, though confufedly, from mixing with one another. When the fpeculum was turned round horizontally, fo that its inclination to the incident rays might be greater, the diftance of the chart remaining the fame (by being drawn round in a circle), the New Process for making Vegetable Lakes. $55 the fpots and fringes evidently were diftended in breadth. I have endeavoured to exhibit the fun's image, as mottled with fringes or rings and fpots, in fig. 5. Obfervation 4-I placed the fpeculum behind a fcreen with a hole in it, through which were let pafs the homogeneal rays of the fun, feparated by refraction through a prifm; this being turned on its axis, the rays which fell on the fpeculum were changed; the fringes were now of that colour whofe rays fell, and when the rays fhifted, the fringes contracted or dilated, being broadeft in the most flexible rays, and confequently in those whofe flexity is greatest. [To be continued.] III. Enquiries refpecting the Colouring Matter of Vegetables, and the Action of Metallic Subftances and their Oxides upon them; together with a New Process for obtaining Lakes of the most intenfe and folid Colours. Read to the National Inftitute (of France) 15 Vendemiaire, in the Year VI. By the C. GUTTON *. LINNEUS, the great naturalift of the north, bad affirmed that the red colours of ve getables announce the prefence of an acid. It was long ago obferved, that the juice of the violet acquires a beautiful blue fhade in veffels of tin, the ufe of which metal was recommended in difpenfatories for the preparation of violet fyrup; and the original colour of fuch fyrups as had been changed by keeping, was restored by long digeftion in tin. Little attention however was paid to the cause of these phenomena; and our affociate, Berthollet, in his Elements of the Art of Dyeing, had pointed it out no otherwife than by conjecture, when he fuppofed an acid to have combined with the oxide formed at the furface of the tin. Such was the state of our knowledge on this fubject, when, from the striking difference of colour of two preparations of the fame fruit, I undertook to examine the circumftances in which thefe changes take place. I fupprefs the detail of experiments to which I fubjected almost all the acid coloured fruits in fucceffion, fuch as the ftrawberry, the goofeberry, the plum, as well as the petals of flowers, turnfol, fernambouc, turmeric, &c. by treating them comparatively in veffels of glass, of porcelain, of metal, and metallic alloys, or by keeping them in digeftion on plates of metal perfectly cleaned, or upon metallic oxides. I fhall confine myself at present to fuch results as may improve the theory of vegetable colours, or afford fome ufeful applications to the processes of the arts. Thefe experiments prove that the red colour of fruits is manifeftly owing to the real action of their peculiar acid upon their colouring matter. That tin, when it brightens or restores the colour of violets, does nothing more than refume, by fuperior affinity, the acid which had caufed it to turn red. That tin or its oxide is not, as has hitherto been thought, the only metal which exercifes * Translated from La Decade philof. litt. et politique, No. II. An VI. (Jan. 1798.) X 2 this |