Produced by Gordon Keener
This eBook includes 5 papers or speeches by James Clerk Maxwell.
The contents are:
Foramen Centrale
Theory of Compound Colours
Poinsot's Theory
Address to the Mathematical
Introductory Lecture
On the Unequal Sensibility of the Foramen Centrale to Light ofdifferent Colours.
James Clerk Maxwell
[From the Report of the British Association, 1856.]
When observing the spectrum formed by looking at a long ve rtical slitthrough a simple prism, I noticed an elongated dark spot running upand down in the blue, and following the motion of the eye as it movedup and down the spectrum, but refusing to pass out of the blue intothe other colours. It was plain that the spot belonged both to theeye and to the blue part of the spectrum. The result to which I havecome is, that the appearance is due to the yellow spot on the retina,commonly called the Foramen Centrale of Soemmering. The mostconvenient method of observing the spot is by presenting to the eye innot too rapid succession, blue and yellow glasses, or, still better,allowing blue and yellow papers to revolve slowly before the eye. Inthis way the spot is seen in the blue. It fades rapidly, but isrenewed every time the yellow comes in to relieve the effect of theblue. By using a Nicol's prism along with this apparatus, the brushesof Haidinger are well seen in connexion with the spot, and the fact ofthe brushes being the spot analysed by polarized light becomesevident. If we look steadily at an object behind a series of brightbars which move in front of it, we shall see a curious bending of thebars as they come up to the place of the yellow spot. The part whichcomes over the spot seems to start in advance of the rest of the bar,and this would seem to indicate a greater rapidity of sensation at theyellow spot than in the surrounding retina. But I find the experimentdifficult, and I hope for better results from more accurate observers.
On the Theory of Compound Colours with reference to Mixtures of
Blue and Yellow Light.
James Clerk Maxwell
[From the Report of the British Association, 1856.]
When we mix together blue and yellow paint, we obtain green paint.This fact is well known to all who have handled colours; and it isuniversally admitted that blue and yellow make green. Red, yellow,and blue, being the primary colours among painters, green is regardedas a secondary colour, arising from the mixture of blue and yellow.Newton, however, found that the green of the spectrum was not the samething as the mixture of two colours of the spectrum, for such amixture could be separated by the prism, while the green of thespectrum resisted further decomposition. But still it was believedthat yellow and blue would make a green, though not that of thespectrum. As far as I am aware, the first experiment on the subjectis that of M. Plateau, who, before 1819, made a disc with alternatesectors of prussian blue and gamboge, and observed that, whenspinning, the resultant tint was not green, but a neutral gray,inclining sometimes to yellow or blue, but never to green.Prof. J. D. Forbes of Edinburgh made similar experiments in 1849, withthe same result. Prof. Helmholtz of Konigsberg, to whom we owe themost complete investigation on visible colour, has given the trueexplanation of this phenomenon. The result of mixing two colouredpowders is not by any means the same as mixing