Accidental and Complementary Colours. 215 



13. Let a number of tubes of bright coloured paper be 

 formed by wrapping the paper once round upon a cylinder 

 of wood, the edges of the paper being secured with paste ; 

 the coloured side of the paper forming the interior of the 

 tubes. The dimensions may be about ten inches in length 

 and one inch in diameter. If one of these tubes be E^pplied 

 to one eye, the other being closed, and the white ceiling be 

 observed, the white circular spot will immediately become 

 the accidental colour of that of the tube. 



Suppose a blue tube be employed, a circular orange 

 coloured spot will be seen upon the ceiling. Decomposition 

 results as before : the white light from the ceiling enters 

 the tube ; the blue is retained, and the red and yellow rays 

 enter the eye and produce the impression of orange. 



14. Some beautiful illusions may be contrived and ex- 

 plained on the principle of chromatic attraction. (1.) Pro- 

 vide several pieces of paste-board about seven inches in 

 length, and about four inches in breadth. Cover one side 

 of each board with coloured paper, and arrange them in 

 pairs, such as green and red ; orange and blue ; indigo and 

 yellow, &c. Take one pair, and place them back to back ; 

 curve them a little outwards at each end, and place the 

 other ends of one pair upon the top of the nose extending up 

 the forehead : look at the white ceiling along the coloured 

 boards in such a manner as that the right eye shall not see 

 the colour along which the left eye views the ceiling, and 

 vice versa. If the pair of colours be well chosen, that is, 

 complementary to each other, the colours will appear 

 vividly depicted upon the ceiling, and they will have changed 

 •places. Thus, suppose the boards be indigo for the right 

 eye, and yellow for the left, the ceiling will be divided into 

 two parts ; the division being mid-way between the two 

 eyes ; on the right hand will be a flood of yellow light, and 

 on the left hand an equal portion of indigo. The observa- 

 tion is best made with the back to the window, and the 

 observer should be seated, the head thrown back and kept 

 steady. 



(2.) Let one of the boards be covered with white paper 

 and another with black. Cut out two disks, about half 

 an inch in diameter each, one white and another black : 

 place the white disk on the centre of the black board, and 



