J 74 



NA rURE 



[June 24, 1897 



For instance, if the Ryves ring is only visible in direct and 

 l)rilliant sunshine, that will be evidence in favour of the shadow 



theory. II' it is equally visible when the sun is not actually 



shining, but when the sky is illuminated by numerous white 



clouds, that will be evidence of the grass theory. 



I may add that our observations were made from the west, and 



at noon. C. V. Bovs. 



An Edinburgh Record of the Indian Earthquake. 



A VERY interesting record of the recent earthquake at 

 Calcutta is shown by the photographic apparatus of the bifilar 

 pendulum of this observatory. A few very slight preliminary 

 tremors commenced June il, at ajh. i8m. G.M.T.', and lasted 

 for ten minutes. Violent oscillations then commenced suddenly, 

 and lasted to Tune 12, oh. 33m., after which slighter tremors 



continued up to ih. 12m. The oscillations can be traced fully 

 six times the measure of sensitiveness of the instrument on one 

 side of the normal line, and four times on the other, which are 

 together equivalent to a tilt of the supporting frame of about 

 twenty seconds of arc. I enclose a negative of the original 

 photograph, which, however, does not show all the minor 

 details of the effect produced. Thomas Heath. 



Royal Observatory, Edinburgh, June 15. 



Subjective Transformations of Colour. 



Mr. Shelfori) Bidwell's experiments, described in Nature 

 of June 10 (p. 128), remind me of a phenomenon which can be 

 very easily demonstrated. 



A disc is arranged so that a small sect or, about one-sixth, is 

 of a bright colour, while the remaining portion is white. 

 If this be rotated slowly, the coloured sector appears to be 

 followed by a ghost of the complementary colour : on quicken- 

 ing the rotation, the original colour is lost, and the whole disc 

 appears to be of the complementary colour ; but if the rotation 

 be further quickened until flickering ceases, the original colour 

 again predominates. In this way emerald-green may appear 

 to change to pink, or crimson-lake to green. 



This seems to be another instance in which the negative after- 

 sensation is stronger than the original sensation. For the success 

 of the experiment it is probably necessary so to adjust the 

 rotation that (<z) the negative sensation has a longer duration 

 than the original sensation, and {b) the next original stimulus 

 shall follow before the after- sensation has entirely faded. If 

 the rotation be too rapid, the negative sensation has not time to 

 develop, and only the original colour is seen. 



Mason College, June 12. F. J. Allen. 



Planetary Orbits, illustrated by a Rolling Ball. 



The interesting article in Nature, April 29, by R. W. 

 Wood, on the orbits of a steel ball about a magnet pole, suggests 

 to me that it may interest some of your readers to hear of another 

 plan for showing these orbits, which, but for the slight resistance 

 of the air, is very nearly theoretically accurate, and in which the 

 proper initial velocities are easily produced. 



The plan consists of causing a true steel ball — of, say, one inch 

 or more in diameter — to roll on a hard and smooth surface of the 

 proper form. The surface on which the centre of the ball 

 moves is formed by the revolution of a part of a rectangular 

 hyperbola about a vertical asymptote, and the real surface on 



NO 1443, VOL. 56] 



which the ball rolls is, of course, a distance equal to the radius- 

 of the ball from this imaginary surface at all points. 



Fig. I is a vertical section, on a very small scale, in which, 

 the dotted lines show the rectangular hyperbola and its vertical 

 and horizontal asymptotes, and the full curved lines show the 

 actual surface on which the ball rolls ; the interval between the 

 dotted and full curved lines being equal to the radius of the balL 



/■/c /. 



Fig. 2 is a projection on a horizontal plane showing contours 

 of the hyperbolic surface at equal vertical intervals in dotted 

 lines, and also one of the orbits referred to below in full lines. 



Suppose the ball to be allowed to roll from the level of the 

 horizontal asymptote, and directed so as not to come too near the 

 centre (c) of the surface, it will describe a path whose projection 

 on a horizontal plane is very nearly a parai)ola ; and — as in the 

 imaginary case of a comet coming from an infinite distance 

 without initial velocity — its energy will be accurately in the 

 inverse proportion to its horizontal distance from the centre of 

 attraction, because the reciprocal of its distance from the centre 

 is proportional to the vertical distance it has fallen, by the well- 

 known property of the rectangular hyperbola. If the ball be 

 allowed to roll from a point higher than the horizontal asymptote, 

 it will describe a hyperbola ; and if fron: a lower point, an ellipse. 



To produce an ellipse having any given axis-major as a b. 

 Fig. 2, let the ball roll from a point u. on the hyperbolic surface, 

 whose distance from the centre c is equal to a b, and let it be 

 directed with a straight-edge till it touches the desired orbit. 



If we could neglect the resistance of the air and all other small 

 resistances to the rolling of the ball, the actual energy, and 

 therefore the velocity of the ball would be precisely what they 

 ought to be to illustrate planetary motion ; the direction of motion 

 not being generally horizontal, however, and the orbit not exactly 

 in one plane. 



The fact that a part of a rolling ball's energy is rotational andl 

 part translational does not vitiate the experiment, because the 

 proportion of the one part of the energy to the other remains- 

 constant. 



In the case of a steel ball attracted by a magnet pole, the 

 acceleration would appear to be inversely as some higher power 

 of the distance than the square. In the case explained above, 

 the acceleration is very nearly inversely as the square of the 

 horizontal distance from the centre, and only differs from this 

 proportion in being less, as the actual distance traversed by the 

 ball exceeds its horizontal component, at any point ; which may 

 be a very small percentage. Geo. Romanes. 



Craigknowe, Slateford, Midlothian. 



