Five of Maxwell's Papers

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The Project Gutenberg EBook of Five of Maxwell's Papers, by James Clerk Maxwell (#1 in our series by James ClerkMaxwell)Copyright laws are changing all over the world. Be sure to check the copyright laws for your country before downloadingor redistributing this or any other Project Gutenberg eBook.This header should be the first thing seen when viewing this Project Gutenberg file. Please do not remove it. Do notchange or edit the header without written permission.Please read the "legal small print," and other information about the eBook and Project Gutenberg at the bottom of thisfile. Included is important information about your specific rights and restrictions in how the file may be used. You can alsofind out about how to make a donation to Project Gutenberg, and how to get involved.**Welcome To The World of Free Plain Vanilla Electronic Texts****eBooks Readable By Both Humans and By Computers, Since 1971*******These eBooks Were Prepared By Thousands of Volunteers!*****Title: Five of Maxwell's PapersAuthor: James Clerk MaxwellRelease Date: January, 2004 [EBook #4908] [Yes, we are more than one year ahead of schedule] [This file was firstposted on March 24, 2002]Edition: 10Language: English*** START OF THE PROJECT GUTENBERG EBOOK, FIVE OF MAXWELL'S PAPERS ***This eBook was produced by Gordon Keener.This eBook includes 5 papers or speeches by James Clerk Maxwell. Each is separated by three asterisks ('***').The contents are: Foramen Centrale Theory ...

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The Project Gutenberg EBook of Five of Maxwell'sPapers, by James Clerk Maxwell (#1 in our seriesby James Clerk Maxwell)sCuorpey triog chth leacwk st haer ec cohpayrniggihnt gl aawll so fvoerr ytohue r wcooruldn.t rByebefore downloading or redistributing this or anyother Project Gutenberg eBook.vTiheiws inhge atdhiesr Psrhoojeulcdt bGeu ttehne bfierrsgt tfihlien. gP lseeaesne wdhoe nnotremove it. Do not change or edit the headerwithout written permission.Please read the "legal small print," and otherinformation about the eBook and ProjectGutenberg at the bottom of this file. Included isimportant information about your specific rights andrestrictions in how the file may be used. You canalso find out about how to make a donation toProject Gutenberg, and how to get involved.**Welcome To The World of Free Plain VanillaElectronic Texts****eBooks Readable By Both Humans and ByComputers, Since 1971*******These eBooks Were Prepared By Thousandsof Volunteers!*****Title: Five of Maxwell's Papers

Author: James Clerk MaxwellRelease Date: January, 2004 [EBook #4908] [Yes,we are more than one year ahead of schedule][This file was first posted on March 24, 2002]Edition: 10Language: English*E**B OSTOAK,R TF IOVEF TOHF E MPARXOWJEELCLT' S GPUATPEENRBSE *R**GThis eBook was produced by Gordon Keener.This eBook includes 5 papers or speeches byJames Clerk Maxwell. Each is separated by threeasterisks ('***').The contents are: Foramen Centrale Theory of Compound Colours Poinsot's Theory Address to the Mathematical Introductory Lecture***COen ntthrael eU tnoe qLiugahl t Soef ndsififbeilirtey nto f Ctohleo uFrosr.amen

James Clerk Maxwell[From the Report of the British Association, 1856.]When observing the spectrum formed by looking ata long vertical slit through a simple prism, I noticedan elongated dark spot running up and down in theblue, and following the motion of the eye as itmoved up and down the spectrum, but refusing topass out of the blue into the other colours. It wasplain that the spot belonged both to the eye and tothe blue part of the spectrum. The result to which Ihave come is, that the appearance is due to theyellow spot on the retina, commonly called theForamen Centrale of Soemmering. The mostconvenient method of observing the spot is bypresenting to the eye in not too rapid succession,blue and yellow glasses, or, still better, allowingblue and yellow papers to revolve slowly before theeye. In this way the spot is seen in the blue. Itfades rapidly, but is renewed every time the yellowcomes in to relieve the effect of the blue. By usinga Nicol's prism along with this apparatus, thebrushes of Haidinger are well seen in connexionwith the spot, and the fact of the brushes being thespot analysed by polarized light becomes evident.If we look steadily at an object behind a series ofbright bars which move in front of it, we shall see acurious bending of the bars as they come up to theplace of the yellow spot. The part which comesover the spot seems to start in advance of the restof the bar, and this would seem to indicate a

ignr tehate esr urrarpoiudintdyi nofg sreetninsaa.t ioBnu ta It ftihned ythelel oewx pseproitm tehnatndifficult, and I hope for better results from moreaccurate observers.***On the Theory of Compound Colours withreference to Mixtures ofBlue and Yellow Light.James Clerk Maxwell[From the Report of the British Association, 1856.]When we mix together blue and yellow paint, weobtain green paint. This fact is well known to allwho have handled colours; and it is universallyadmitted that blue and yellow make green. Red,yellow, and blue, being the primary colours amongpainters, green is regarded as a secondary colour,arising from the mixture of blue and yellow.Newton, however, found that the green of thespectrum was not the same thing as the mixture oftwo colours of the spectrum, for such a mixturecould be separated by the prism, while the green ofthe spectrum resisted further decomposition. Butstill it was believed that yellow and blue wouldmake a green, though not that of the spectrum. Asfar as I am aware, the first experiment on thesubject is that of M. Plateau, who, before 1819,made a disc with alternate sectors of prussian blueand gamboge, and observed that, when spinning,

the resultant tint was not green, but a neutral gray,inclining sometimes to yellow or blue, but never togreen. Prof. J. D. Forbes of Edinburgh madesimilar experiments in 1849, with the same result.Prof. Helmholtz of Konigsberg, to whom we owethe most complete investigation on visible colour,has given the true explanation of this phenomenon.The result of mixing two coloured powders is not byany means the same as mixing the beams of lightwhich flow from each separately. In the latter casewe receive all the light which comes either from theone powder or the other. In the former, much ofthe light coming from one powder falls on particlesof the other, and we receive only that portion whichhas escaped absorption by one or other. Thus thelight coming from a mixture of blue and yellowpowder, consists partly of light coming directly fromblue particles or yellow particles, and partly of lightacted on by both blue and yellow particles. Thislatter light is green, since the blue stops the red,yellow, and orange, and the yellow stops the blueand violet. I have made experiments on themixture of blue and yellow light—by rapid rotation,by combined reflexion and transmission, by viewingthem out of focus, in stripes, at a great distance,by throwing the colours of the spectrum on ascreen, and by receiving them into the eye directly;and I have arranged a portable apparatus by whichany one may see the result of this or any othermixture of the colours of the spectrum. In all thesecases blue and yellow do not make green. I havealso made experiments on the mixture of colouredpowders. Those which I used principally were"mineral blue" (from copper) and "chrome-yellow."

Other blue and yellow pigments gave curiousresults, but it was more difficult to make themixtures, and the greens were less uniform in tint.The mixtures of these colours were made byweight, and were painted on discs of paper, whichwere afterwards treated in the manner described inmy paper "On Colour as perceived by the Eye," inthe Transactions of the Royal Society ofEdinburgh, Vol. XXI. Part 2. The visible effect ofthe colour is estimated in terms of the standard-coloured papers:—vermilion (V), ultramarine (U),and emerald-green (E). The accuracy of theresults, and their significance, can be bestunderstood by referring to the paper beforementioned. I shall denote mineral blue by B, andchrome-yellow by Y; and B3 Y5 means a mixtureof three parts blue and five parts yellow. Given Colour. Standard Colours. Coefficient V. U. E. of brightness.B8 , 100 = 2 36 7 ………… 45 B7 Y1, 100 = 1 1817 ………… 37 B6 Y2, 100 = 4 11 34 ………… 491B 54 0Y 3…, 1…0…0 =… 95 56 4B03 …Y5…, 1…00… =5 24 2 B-4 2 Y444, 1…00… =… 1…564 B2 Y6, 100 = 35 -10 51 ………… 76 B1 Y7,100 = 64 -19 64 ………… 109 Y8, 100 = 180 -27124 ………… 277The columns V, U, E give the proportions of thestandard colours which are equivalent to 100 of thegiven colour; and the sum of V, U, E gives a

coefficient, which gives a general idea of thebrightness. It will be seen that the first admixture ofyellow diminishes the brightness of the blue. Thenegative values of U indicate that a mixture of V,U, and E cannot be made equivalent to the givencolour. The experiments from which these resultswere taken had the negative values transferred tothe other side of the equation. They were all madeby means of the colour-top, and were verified byrepetition at different times. It may be necessary toremark, in conclusion, with reference to the modeof registering visible colours in terms of threearbitrary standard colours, that it proceeds uponthat theory of three primary elements in thesensation of colour, which treats the investigationof the laws of visible colour as a branch of humanphysiology, incapable of being deduced from thelaws of light itself, as set forth in physical optics. Ittakes advantage of the methods of optics to studyvision itself; and its appeal is not to physicalprinciples, but to our consciousness of our ownsensations.*** On an Instrument to illustrate Poinsot's Theoryof Rotation.James Clerk Maxwell[From the Report of the British Association, 1856.]In studying the rotation of a solid body according toPoinsot's method, we have to consider thesuccessive positions of the instantaneous axis of

rotation with reference both to directions fixed inspace and axes assumed in the moving body. Thepaths traced out by the pole of this axis on theinvariable plane and on the central ellipsoid forminteresting subjects of mathematical investigation.But when we attempt to follow with our eye themotion of a rotating body, we find it difficult todetermine through what point of the body theinstantaneous axis passes at any time,—and todetermine its path must be still more difficult. Ihave endeavoured to render visible the path of theinstantaneous axis, and to vary the circumstancesof motion, by means of a top of the same kind asthat used by Mr Elliot, to illustrate precession*. Thebody of the instrument is a hollow cone of wood,rising from a ring, 7 inches in diameter and 1 inchthick. An iron axis, 8 inches long, screws into thevertex of the cone. The lower extremity has a pointof hard steel, which rests in an agate cup, andforms the support of the instrument. An iron nut,three ounces in weight, is made to screw on theaxis, and to be fixed at any point; and in thewooden ring are screwed four bolts, of threeounces, working horizontally, and four bolts, of oneounce, working vertically. On the upper part of theaxis is placed a disc of card, on which are drawnfour concentric rings. Each ring is divided into fourquadrants, which are coloured red, yellow, green,and blue. The spaces between the rings are white.When the top is in motion, it is easy to see in whichquadrant the instantaneous axis is at any momentand the distance between it and the axis of theinstrument; and we observe,—1st. That theinstantaneous axis travels in a closed curve, and

returns to its original position in the body. 2ndly.That by working the vertical bolts, we can make theaxis of the instrument the centre of this closedcurve. It will then be one of the principal axes ofinertia. 3rdly. That, by working the nut on the axis,we can make the order of colours either red,yellow, green, blue, or the reverse. When the orderof colours is in the same direction as the rotation, itindicates that the axis of the instrument is that ofgreatest moment of inertia. 4thly. That if we screwthe two pairs of opposite horizontal bolts todifferent distances from the axis, the path of theinstantaneous pole will no longer be equidistantfrom the axis, but will describe an ellipse, whoselonger axis is in the direction of the mean axis ofthe instrument. 5thly. That if we now make one ofthe two horizontal axes less and the other greaterthan the vertical axis, the instantaneous pole willseparate from the axis of the instrument, and theaxis will incline more and more till the spinning canno longer go on, on account of the obliquity. It iseasy to see that, by attending to the laws ofmotion, we may produce any of the above effectsat pleasure, and illustrate many differentpropositions by means of the same instrument.* Transactions of the Royal Scottish Society ofArts, 1855.***Address to the Mathematical and Physical SectionsoAfs tshoec iBatriitoisn.h

James Clerk Maxwell[From the British Association Report, Vol. XL.][Liverpool, September 15, 1870.]At several of the recent Meetings of the BritishAssociation the varied and important business ofthe Mathematical and Physical Section has beenintroduced by an Address, the subject of which hasbeen left to the selection of the President for thetime being. The perplexing duty of choosing asubject has not, however, fallen to me.Professor Sylvester, the President of Section A atthe Exeter Meeting, gave us a noble vindication ofpure mathematics by laying bare, as it were, thevery working of the mathematical mind, and settingbefore us, not the array of symbols and bracketswhich form the armoury of the mathematician, orthe dry results which are only the monuments ofhis conquests, but the mathematician himself, withall his human faculties directed by his professionalsagacity to the pursuit, apprehension, andexhibition of that ideal harmony which he feels tobe the root of all knowledge, the fountain of allpleasure, and the condition of all action. Themathematician has, above all things, an eye forsymmetry; and Professor Sylvester has not onlyrecognized the symmetry formed by thecombination of his own subject with those of theformer Presidents, but has pointed out the dutiesof his successor in the following characteristic note:

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