AND THE THEORY OF LIGHT. 87 



Ewperiments. 



Take a card and describe on it a semi-circle ; it may be 

 either cut out of, or painted on it. {See plate V. fig. 6.) 

 Fix it on the machine, so that the diameter a b may be per- 

 fectly in a line with the axis of motion. When in motion the 

 semi-circle will appear a circle. There will be a line, a h, 

 of bright colour along the diameter, equal in breadth to 

 the thickness of the card, if the semi-circle is cut out. 

 Should the diameter a 6 be removed in the smallest degree 

 from the centre of motion, the diameter of the generated 

 figure will show various shades of purple and green. On 

 each side of the diameter, the semi-circles of which the 

 generated circle appears to be composed, will be coloured, 

 not unlike a peach, beginning at the diameter with purple, 

 then approaching to yellow, and verging to green towards 

 the circumference. 



This is another demonstration of the correctness of my 

 views with regard to the change of colour which takes 

 place on reversing the motion of some of the plane figures, 

 for in this experiment the light on each semi- circle visibly 

 proceeds from two different quarters. One, however, is 

 more apt to be struck with the phenomenon when hori- 

 zontal motion is employed. 



135. To vary this experiment, in place of a half, cut out 

 a whole circle. In this case there will be a black line 

 along the diameter where there was formerly a bright line, 

 but it will be shaded off on each side towards the circum- 

 ference as before. When the figure thus generated is 

 minutely examined it will be seen to consist of a hollow 

 globe, and the shadows from the edges of the card will 

 distinctly appear on the back of the generated figure. 

 The shadow on the aerial figure formed by the card is very 

 interesting. If a circle painted black be used this ap- 

 pearance will be wanting. 



