12 REPORT — 1856. 



On the Unequal Sensibility of the Foramen Centrale to Light of 

 different Colours. By J. C. Maxwell. 

 When observing the spectrum formed by looking at a long vertical slit through a 

 simple prism, I noticed an elongated dark spot running up and down in the blue, 

 and following the motion of the eye as it moved up and down the spectrum, but 

 refusing to pass out of the blue into the other colours. It was plain that the spot 

 belonged both to the eye and to the blue part of the spectrum. The result to which 

 I have come is, that the appearance is due to the yellow spot on the retina, com- 

 monly called the Foramen Centrale of Soemmering. The most convenient method of 

 observing the spot is by presenting to the eye in not too rapid succession, blue and 

 yellow glasses, or, still better, allowing blue and 5'ellow papers to revolve slowly 

 before the eye. In this way the spot is seen in the blue. It fades rapidly, but is 

 renewed every time the yellow comes in to relieve the effect of the blue. By using a 

 Nicol's prism along with this apparatus, the brushes of Haidinger are well seen in 

 connexion with the spot, and the fact of the brushes being the spot analysed by po- 

 larized light becomes evident. If we look steadily at an object behind a series 

 of bright bars which move in front of it, we shall see a curious bending of the bars 

 as they come up to the place of the yellow spot. The part which comes 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 the yellow spot than in the surrounding retina. 

 But I find the experiment diflScult, and I hope for better results from more accurate 

 observers. 



On a Method of Drawing the Theoretical Forms 0/ Faraday's Lines of Force 



without Calculation. By J. C. Maxwell. 

 The method applies more particularly to those cases in which the lines are entirely 

 parallel to one plane, such as the lines of electric currents in a thin plate, or those 

 round a system of parallel electric currents. In such cases, if we know the forms 

 of the lines of force in any two cases, we may combine them by simple addition of 

 the functions on which the equations of the lines depend. Thus the system of hues 

 in a uniform magnetic field is a series of parallel straight lines at equal intervals, and 

 that for an infinite straight electric current perpendicular to the paper is a series of 

 concentric circles whose radii are in geometric progression. Having drawn these two 

 sets of lines on two separate sheets of paper, and laid a third piece above, draw a 

 third set of lines through the intersections of the first and second sets. This will 

 be the system of lines in a uniform field disturbed by an electric current, llie most 

 interesting cases are those of uniform fields disturbed by a small magnet. If we 

 draw a circle of any diameter with the magnet for centre, and join those points in 

 which the circle cuts the lines of force, the straight lines so drawn will be parallel and 

 equidistant ; and it is easily shown that they represent the actual lines of force in a 

 paramagnetic, diamagnetic, or crystallized body, according to the nature of the ori- 

 ginal lines, the size of the circle, &c. No one can study Faraday's researches without 

 wishing to see the forms of the lines of force. This method, therefore, by which 

 they may be easily drawn, is recommended to the notice of electrical students. 



On the Theory of Compound Colours with reference to Mixtures of Blue 

 and Yellow Light. By J. C. Maxwell. 

 When we mix together blue and yellow paint, we obtain green paint. This fact 

 is well known to all who have ever handled colours ; and it is universally admitted 

 that blue and yellow make green. Red, yellow, and blue, being the primary colours 

 among painters, green is regarded as a secondary colour, arising from the mixture of 

 blue and yellow. Newton, however, found that the green of the spectrum was not 

 the same thing as the mixture of two colours of the spectrum, for such a mixture 

 could be separated by the prism, while the green of the spectrum resisted further de- 

 composition. But still it was believed that yellow and blue would make a green, 

 though not that of the spectrum As far as I am aware, the first experiment on the 

 subject is that of M. Plateau, who, before 1819, made a disc with alternate sectors 

 of Prussian blue and gamboge, and observed that, when spinning, the resultant 



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