84 lUR, J, SMITH ON THE ORIGIN OF COLOUR 



PERPENDICULAE MOTION. 



Experiments in Imitation of Transparent or Crystalline 

 Solids. 



126. The experiments hitherto described were all made 

 by employing horizontal motion. By changing the mo- 

 tion from horizontal to perpendicular, I have endeavoured 

 to imitate solids, and show other effects seen in the pas- 

 sage of light through transparent media. 



It will scarcely be necessary to premise that the revolu- 

 tion of a parallelogram by perpendicular motion produces 

 a cylinder, that the revolution of a triangle produces a 

 cone or a double cone, and that the revolution of a semi- 

 circle produces a sphere. In fact this class of experi- 

 ments affords an immense variety of figures which an 

 ingenious and inventive mind may diversify to any extent. 



127. Those who understand the principles already ex- 

 plained will easily understand what is to follow. The 

 principles are entirely the same, but in some of the 

 experiments colour, from the nature of the generated 

 figure, is. seen more distinctly than even in the former 

 experiments to be caused by very sensible and easily 

 counted beats. Although the principles involved are 

 extremely simple, it may be advisable before proceeding 

 further to advert for a moment to 



The Theory of Colour by Perpendicular Motion. 



128. Let us take in our hand a white card A B, {see 

 plate V. Jiff 5,) made of Bristol board, and let us examine 

 what we see when we hold it by one end. If it is kept 

 between the eye and the floor the whole of the card 

 appears white, and the space around it dark. Divide the 

 end into two equal parts at a, place the middle point 

 exactly on the pivot of the wheel, and make it rotate. No 



