of Colour and Visible Motion. 123 



very difficult to avoid abrasion, and consequent slight irregu- 

 larities in the motions of the slider, since the springs pressing 

 down these sliders must be moderately strong to cause them 

 to promptly follow all the alterations in curvature of the dif- 

 ferent parts of the roller. 



Fig. 5 shows (reduced to a scale of one sixth) the roller 

 as it came from the lathe before being cut to fit the tem- 

 plates ; and figs. 6, 7, 8, 9, 10, 11 the sections at AB, CD, 

 E F, G H, I J, K L (fig. 5) one quarter full size. Small 

 circles have been drawn representing the little friction- 

 wheel, and at such distances apart that in all cases the time 

 taken for the wheel to pass from one position to the next is 

 constant and equal to the forty-eighth part of the period of the 

 revolution of the roller. 



We used six sliders, a, j3, 7, e, f, n (fig. 1), one of which is 

 shown enlarged in fig. 4. Each of these sliders could be 

 moved longitudinally, parallel to the axis of the roller, along 

 two stout iron bars L M, P (fig. 1), in order to alter the 

 amplitude of the swings. Sliders a. and e could be made to 

 rest anywhere between B and F, /3 and J anywhere between 

 F and G, and 7 and rj anywhere between Gr and 0. Each 

 slider carried at its upper end a large pulley made to move 

 very easily (fig. 4) ; and the three pulleys of a, /3, 7 were 

 always in the plane of the fixed pulleys T, U, V attached 

 above to the wooden frame. A fine inextensible cord passing 

 round the movable pulleys a, /3, 7, and the fixed pulleys, S, T, 

 U, Y , was fixed at one extremity to the pulley Y (the pur- 

 pose of which was simply to adjust the length of the cord) ; 

 and at the other end, where it hung vertically, it was attached 

 to the top of the glass plate abed, from the bottom of which 

 hung a weight in a pail of water to damp the motion of the 

 weight. This system then gave to the glass plate the sum of 

 the motions of the sliders a, ft, and 7. A similar cord over 

 e, f, 7] and W, X, Y, Z, together with the cord passing over 

 the fixed pulley Q, and to which also hangs a weight in a pail 

 of water, gives to the glass plate a horizontal motion equal to 

 the sum of the harmonic motions of the sliders e, £, 77. It is 

 evident, therefore, that to a circular patch stuck on the centre 

 of the glass we were able to give motions compounded of the 

 above -harmonic motions perpendicular to one another, and 

 by projection, by means of an electric light or heliostat, to 

 cause this motion to appear like that of a large black ball 

 rolling about on a white or coloured background. 



Even still greater variety could have been imparted to the 

 figures by the metal rods L M, P (on which the sliders 

 moved) having a motion at right angles to the radius of the 

 roller. This could easily be arranged if the ends of the metal 



