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is, supposing white light to consist of the motion of an ether, 
blackness to consist of an entire absence of motion, then a 
certain colour, blue, red, or yellow, will be produced by the 
alternate action of the light and the shadow. The Author 
used shadow in the positive sense as the sensation was 
positive. 
On pursuing the inquiry, he first caused a small parallelo- 
gram cut in card board to revolve over a black surface with a 
rapidity which he considered equal to the vibration of light. 
By this motion he obtained a distinct blue, while at another 
time in different weather he obtained a purple. He then 
made a disc with several concentric rings, which he painted 
respectively f, |, and ^ black, leaving the remainder white, 
and on making this disc revolve the rings became completely 
coloured. There was no appearance of any black or white. 
In a bright day with white clouds in the sky the rings were 
coloured respectively a light yellowish green, two different 
shades of purple, and a pink. By using discs of a great 
variety of shapes and different proportions of white and black, 
the Author said that he produced sucessively or together all 
the colours of the rainbow, although he had not yet arrived at 
the exact arithmetical determination of the amount of light 
and shade needful for each. 
These experiments were made before the Society by the 
light of a paraffin oil lamp with a reflector. The Author said 
that they were much more brilliant by sunlight. 
There was another set of experiments which the Author 
considered as very effective, and especially as being easily 
made and described, but requiring strong sunshine to show 
them. These were made by casting a shadow of a par- 
ticular figure on a white wall or on a sheet of paper, so as to 
produce alternate beats of light and shadow when put in 
revolution. The figure became coloured of different shades, 
and because these could be seen on the wall, like the spectrum 
from the prism, he called them spectra by reflection. 
