128 



NATURE 



[June io, 1897 



The moment of inertia of the rod was determined, both 

 by calculation and experiment, with very satisfactory 

 agreement, and all the linear measurements of the 

 apparatus, and of the distances, were made very care- 

 fully by horizontal and vertical cathetometers. Ingenious 

 reflexion devices were used for measurements, which were 

 made through the walls of the receiver. 



Dr. Braun used both the deflexion method of Caven- 

 dish, and the oscillation method first used by Reich, to 

 whom it was suggested by Forbes. In the deflexion 

 method the attracting masses are placed outside the 

 case, in such positions that their pulls on the attracted 

 masses twist the rod round. The deflexion is observed, 

 and the value of the corresponding torsion couple is 

 determined by the time of vibration of the system. When 

 the rod is deflected it does not, of course, take up its new 

 position in a " dead beat " manner, but oscillates about it. 

 The usual method of determining the centre of swing has 

 been to observe successive elongations or turning-points, 

 and by combining these in threes to eliminate the effect 

 of decrement, and so to deduce the centre. But Dr. 

 Braun found the centre more accurately by observing the 

 times of transit of several scale divisions near the centre, 

 in both directions. By interpolation he could determine 

 the point about which the ti'me of oscillation in either 

 direction was the same, and this was taken as the centre 

 of swing. The deflexion observed was about 13 divisions 

 of the scale. The times of transit were registered on a 

 chronograph. 



The wire showed a certain amount of elastic after- 

 action, and by subsidiary experiments on a similar wire 

 this was as far as possible allowed for. 



In the oscillation method the attracting masses are 

 placed in a line with the torsion-rod, one at each end. 

 Their attractions then act, not to deflect the rod, but to 

 increase the restoring force, and so to lessen the time of 

 vibration. The attraction is determined by comparing 

 the times of vibration when the masses are in position, and 

 when they are removed, or when they are placed so that the 

 line joining their centres is at right angles to the rod. 



The time of vibration observed by Dr. Braun was 

 about 1275 seconds, and when the masses were put in 

 position this was altered by about 46 seconds. 



The results obtained in the years 1892 and 1894 were 

 finally used, and these gave for the mean density of the 

 earth — 



1892 1894 



Deflexion method 5*529 ••• 5 '526 



Oscillation ,, 5"520 ... 5-531 



Giving due weight to the various observations, the final 

 result is practically identical with that of Prof. Boys', viz. : 

 Mean density = 5*52725+ -0012 

 Gravitation constant G = 665786 x lO"" 



J. H. P. 



SUBJECTIVE TRANSFORMATIONS OF 

 COLOUJi. 

 TN a communication to the Royal Society on May 13, 

 -*■ I described some curious experiments, showing how 

 coloured objects might apparently be made to assume 

 tints which were complementary to their actual hues — 

 red, for example, appearing as green or greenish-blue, 

 and green as pale red. 



The phenomenon depends upon the rapid generation 

 of negative after-images of the kind demonstrated by the 

 familiar experiment with the red " wafer." If a red 

 wafer lying upon a sheet of white paper is looked at 

 steadily for about half a minute, and the gaze is then 

 suddenly transferred to some other part of the paper, a 

 greenish-blue ghost of the wafer will be seen. The por- 

 tion of the retina upon which the red image at first falls 

 becomes fatigued and partially insensible to red light ; it 

 is therefore unable to appreciate the red component of 

 the white light afterwards reflected to it, and the 



NO. 1 44 1, VOL. 56] 



sensation of the complementary colour consequently 

 predominates. 



The new experiments indicate that the preliminary 

 stare may, under certain conditions, be an exceedingly 

 brief one. In a paper published three years ago {Proc. 

 Roy. Soc, vol. Ivi. p. 132) I called attention to an observ- 

 ation indicating that a short period of darkness imparts 

 to the retinal nerves a degree of sensitiveness, which is 

 far above the normal average in the light, and which 

 quickly passes away again under the influence of illu- 

 mination. This peculiar sensitiveness is in fact both 

 acquired and lost in a small fraction of a second, and 

 is therefore very favourable for the rapid production of 

 negative after-images. 



Let two small screens — one black and the other white- 

 be held together in one hand, and arranged so that there 

 may be a triangular gap between them. Let the black 

 screen first cover the paper upon which the wafer is 

 lying ; this will darken a portion of the retina, and render 

 it sensitive. Then let the screens be quickly moved 

 sideways, so that the wafer may for a moment be exposed 

 to view through the gap, the movement .being stopped as 

 soon as the paper is covered by the white screen. A 

 bright but evanescent greenish-blue ghost will succeed 

 the red impression. But 

 the curious thing is that 

 if the illumination is 

 strong, and the screens 

 are moved at the proper 

 speed, no trace of red 

 will be seen at all ; it 

 will appear exactly as if 

 the actual colour of the 

 wafer were greenish-blue. 

 The action of light after 

 a short period of darkness 

 seems to have the power 

 of appreciably diminish- 

 ing the sensibility of the 

 retinal nerve-fibres in a 

 space of time so short, 

 that if the light be coloured 



its colour is not consciously perceived. I am informed that 

 analogous phenomena have been observed in other 

 branches of physiology ; a well-defined reaction some- 

 times occurs when no direct evidence can be detected 

 of the existence of the excitation to which the reaction 

 must have been due. 



By the use of a rotating disc having a black and white * 

 surface and an open sector, as in the annexed figure, the 

 effect can be shown continuously. The disc is made to 

 turn some six or eight times in a second, while its front 

 surface is strongly illuminated either by bright diffused 

 daylight or by a powerful lamp. An incandescent lamp 

 of 32 candle-power at a distance of six inches gives 

 excellent results ; it should be placed opposite the disc, 

 and should be provided with a small tin reflector to pro- 

 tect the eyes from the glare. A red card placed behind 

 the disc is made to appear green, a green card pink, and 

 a blue one yellow, while a black patch painted upon a 

 white ground appears whiter than the ground itself. At 

 the conversazione of the Royal Society on May 19, I 

 exhibited some designs which had been prepared for the 

 purpose of demonstrating the phenomenon in a striking 

 manner. Among them was a picture of a lady with 

 indigo-blue hair, an emerald-green face, and a scarlet 

 gown, who was represented as admiring a violet sun- 

 flower with purple leaves. Seen through the disc the 

 lady's tresses appeared flaxen, her complexion a delicate 

 pink, and her dress a light peacock-blue, while the petals 

 of the sunflower became yellow and its leaves green. 

 Other designs showed equally remarkable transformations 

 of colour. Shelford Bidwell. 



1 A pale brownish-grey tint is better than pure white. 



