no 



SCIENTIFIC RECREATIONS. 



terminates at its lower end with a blunt point of untempered steel ; the 

 cylindrical part of the axis is roughened to encourage the adherence of the 

 string ; the axis is placed between the clamps of a vice, and a plate is put 

 underneath ; we then pull the string firmly with the right hand, and when ' 

 the top is in motion it is separated from the clamps. By pulling the 

 string very powerfully it is possible to obtain a speed of sixty turns in a ^ 

 second, and the movement will be kept up for three quarters of an hour. 



Besides tops, we may make use of different kinds of discs, with an 

 axis rotating between two clamps ; they are moved either by a kind of 

 clock-work, or by the unrolling of a string, like the tops. Generally, how- 

 ever, these contrivances have this inconvenience, that the discs cannot be 

 changed without stopping the instrument, and partly taking it to pieces. 

 On the other hand, we have the advantage of being able to turn them on a 



Fig. 108. Spinning a top with coloured discs. 



vertical plane, so that we can conveniently carry on our experiments before 

 a numerous auditory, which is a more difficult matter with tops. Montigny 

 contrived to obtain the mingling of colours by means of a turning prism, 

 which he caused to throw its shadow on a white screen The Thaumatrope 

 is a small rectangle of cardboard, which is made to rotate on an axis 

 passing through the centres of the longest sides. We shall describe it at 

 greater length when we come to consider a new apparatus known under the 

 name of the Praxinoscope. 



More complicated contrivances have also been constructed on the same 

 principle, by which one may perceive the rotating disc through slits which 

 turn at the same time. We will now describe the construction of some 

 discs invented by Plateau under the name of the Phenakistoscope. These 

 discs are made of strong cardboard, from six to ten inches in diameter 

 (fig. 112), on which a certain number of figures (eight to twelve) are placed 



