108 



SCIENTIFIC RECREATIONS. 



out the whole duration of the period. To verify the truth of this law, we 

 will make use of some discs, such as that represented in fig. 105. The inner- 

 most circle is half white and half black ; the middle circle has two quarters, 



or half its periphery, white, and the outer 

 circle has four eighths' white, the rest being 

 black. If such a disc is turned round, its 

 entire surface will appear grey ; only it is 

 necessary to turn it with sufficient force 

 to produce a continuous effect. The white 

 may also be distributed in other ways, and 

 provided only that on all the circles of the 

 disc the proportion of the angles covered 

 with white is the same, they will always 

 exhibit the same grey colour. Instead of 

 black and white we may make use of different 

 Fig. 1 05. Disc which appears uniformly grey colours, and obtain the same resultant colour 



by reason of its rotation. - , t . 



from all the circles, when the proportion of 



the angles occupied by each of the colours in the different circles is the 

 same. 



If we paint on a disc a coloured star, which is detached from a founda- 

 tion of another colour (fig. 1 06), during the rapid rotation of the disc the 

 centre affects the colour of the star ; the outer circle assumes that of the 



background, and the intermediate parts of the 

 disc present the continuous series of the resultant 

 colours. These results are in accordance with 

 the theory of the mixture of colours. 



Rotative discs, ~hich are so much used in 

 experiments in optical physiology, were em- 

 ployed for the first time by Miisschenbroeck ; 

 the most simple is the top. M. Helmholtz 

 ordinarily uses a brass spinning-top, which fig. 107 

 represents at a third the natural size. It is set 

 in motion by the hand, and its quickness may 

 be ^creased or moderated at will ; but it cannot 

 be made to spin quicker than six rounds in a 

 second ; this motion will be kept up for three or four minutes. Thus, with 

 a feeble movement of rotation, a uniform luminous impression can only be 

 obtained by dividing the disc into four or six sections, on each of which we 

 repeat the same arrangement of colours, light, and shade. If the number of 

 repetitions of the design is less, we obtain, with a bright light, a more or less 

 shot-coloured disc. 



It is easy to place designs on the disc, even when in motion, or to 

 make any desired modification, by superposing on the first disc another 

 disc with sectors, of which we can vary the position by slightly touching it, 

 or even blowing on it, thus producing during the rotation of the disc very 



