30 On the Halo seen around ail Bodies. 
cause of our not seeing objects through it. The continued action 
of light destroys certain combinations which develop colors, and 
thus decomposes them ; but light has no such annihilating power 
as Newton assigned to it. It is the opacity of accumulation 
which hides colored bodies from our sight; light has no power 
to decompose or annihilate instanter ; on the contrary, there are 
certain qualities in the black principle which annihilates light 
itself. 
39. There is an experiment of painting the different colors on 
the periphery of a wheel, and it has become a settled question in 
optics that these colors all blend into a white mass as soon as the 
wheel is in rapid motion. It is known that the duration of im- 
pression, on the organs of vision, are very limited; we cannot 
wonder therefore, that the feeble rays of light which are to give 
an impression of the colored patches on the wheel, should fail in 
doing so. As the rapidity of motion prevents the duration of 
impression of the feebler rays of colored objects, nothing remains 
perceptible to vision but the dense mass of light from the whole 
circumference of the wheel. 
AQ. If on one side of a card—as in the beautiful experiments 
of Dr. Paris—we paint a head, and on the other side a body, by 
making the card turn rapidly, the head appears to be attached to — 
the body. The feeble rays proceeding from that edge or line of 
the circumference which passes before the eye at every half rev- 
olution, are lost in the mass of light which is reflected from the — 
colored and white portions of the card. But if the card be made 
to turn very rapidly, even the figures are hidden, because, as has 
been observed, the duration of impression is very limited. 
Al. A pin-hole at a little distance appears very small, but the 
aperture increases as the distance decreases. It is only when the 
pin-hole is close to the eye, that the true magnifying or lenticular 
power of the halo is recognized. It will be then perceived that 
it completely occupies the whole opening, and that objects which 
are held between the eye and the pin-hole, will all be inverted, 
not only inverted both as to position and movement, but also pos 
sessing a much larger outline than the objects themselves. ‘This 
aperture, therefore, is a lens whose magnifying power is accord- _ 
ing to the size of the aperture and its distance from the eye. 
42. On looking through the halo as it fills up the pin-hole, we 
shall perceive that objects beyond it are very igus 
