ROTATION OF COLOURED DISCS. 71 
lowing heads: 1. Apparent mixture of two or more different 
colours, arranged contiguously, forming hues, tints, shades, 
neutrals, &c.; 2. Contrast of tone; 3. The complementary 
colours; 4. Contrast of colour ; and 5. Mixture by “ soften- 
ing off,” or insensible gradation. 
The requisites for these illustrations are simple and si in 
number. They are a rotating apparatus or colour-top ; 
circular discs of white, black, green, yellow, red, and hes ; 
six heart-shaped scales of the same colours ; five graduated 
scales of white, green, yellow, red, and blue; and a black 
double semizone.* 
Here I would notice, in limine, that the colour-top should 
be spun upon a table placed near the window, and when in 
the act of pulling the string it should be pressed firmly 
down, and not allowed to drop from a height, as is com- 
monly done. The darker the rest of the room the better. 
The table is provided with a cloth, light or dark in colour, 
according to the experiments. These should be performed 
by daylight ; the ight from a white cloud is best, then the 
light from the sun as seen through a white window-blind ; 
that from a blue sky is the worst kind of light that can be 
chosen, hence a bright cloudless day should not be taken. 
A white table-cloth is used in the following experiments. 
1. On the Formation of the Hues, Tints, Shades, Neutrals, 
and Grays. 
The discs here employed are circles of cardboard, either 
white, black, or coloured (figs. 1 to 6), and which, having a 
shit cut completely through from the centre to the circum- 
ference, can be made mutually to overlap one another in 
sectors of any magnitude. 
When sectors of two or more discs of different colours are 
screwed on the colour-top and rotated, the optical composi- 
tion of each colour immediately vanishes, and is replaced by 
an apparent wash of one single colour only, evenly distributed 
on the entire surface of the disc, and which is in reality a 
combination, taking place in the eye itself, of the rays re- 
flected from all the coloured surfaces employed i in the experi- 
ments, thus— 
(Red + blue) = violet or a violet hue. 
(Red + white) = light red or a ¢int of red. 
(Red + black) = dark red or a shade of red. 
* The whole of the apparatus necessary to these experiments, together 
with the discs for producing the kaleidoscopic effects, may be procured of 
the publishers, Messrs. Smith, Beck, and Beck, Opticians, 6, Coleman 
Street, London, under the title of the Kaleidoscopic Colour-top. 
VOL. VII. H 
