0» MR. J. SMITH ON THE ORIGlxN OF COLOUR 



136. Another variety of this experiment may be made 

 by cutting out two semi-circles, making the circumference 

 of the one touch the other^ and fixing the figure so that 

 one of the diameters may be in a line with the axis. {See 

 plate V. fig. y, and coloured diagram, plate II.) The 

 figure is fixed on the axis of the machine at a. 



The appearance is precisely the same as before, as re- 

 gards colour. The form differs in having two semi-circles 

 besides the circle. The semi-circle, which has its diameter 

 fixed on the axis of motion, will produce a circle, the other 

 a semi-circle on each side of the circle. The whole figure 

 when in motion has the appearance of two cylinders, one 

 within the other. The reason is obvious. 



In this figure there are two small circles cut out at a 

 distance (from the centre of each of the larger circles) 

 equal to the radii of the larger circles. These were made 

 to assist the eye in following the motion of the larger 

 circles, and as the figure generated is rather remarkable 

 they have been retained. Observe the difference in colour 

 of these small circles in the outer cylinder and of those 

 in the inner cylinder. In the outer the colour is bluish, 

 in the inner red, showing a bright line up the centre 

 caused by the small bit of Bristol board connecting the 

 two semi-circles, as may be seen by examining the gene- 

 rating figure. {See plate V.fig. 7.) In the inner cylin- 

 der the motion of the small circle is traced receding as far 

 as the eye can follow it. It will be observed that the 

 colour is when receding nearly the same as in the outer 

 cylinder, but partaking more of green. 



137, Take another card, and cut out parallelograms, all 

 of the same or of different breadths. {See plate V.fig. 8.) 

 The parallelograms on the right and left of the axis will 

 be coloured alike, as in the semi-circles. I have often 

 imagined the colour of these to resemble clouds, and the 

 different parallelograms to represent the stratification of 



