110 PHYSICS. 
Light is most intense at its source, and experiences a gradual diminution 
in its intensity as the distance from this source is increased, as is shown by 
the fact of a body becoming less illuminated as it recedes from any radiant. 
The law of this diminution is the same as in the case of heat ; the intensity 
decreases as the square of the distance from the radiant. A body which 
experiences a certain intensity of light at a distance of one foot, will 
receive at the distance of two feet only one fourth of this amount, and at 
the distance of three feet one ninth, &c. 
When light coming from a single luminous point strikes upon an 
opake body, there arises behind this, on the side opposite to the radiant, a 
dark space called a shadow, bounded by a conical surface. If the luminous 
body be of considerable extent, it becomes necessary to distinguish the full 
shadow, or that space receiving no light at all, from the half shadow, or the 
space receiving light from some parts of the luminous body and not from 
others. In pl. 21, fig. 2, let A be a large luminous sphere, and B a smaller 
opake one, then both the full and the half shadows will be conical spaces, 
only of opposite positions; for while the diameter of the full or central 
shadow diminishes with the distance from the luminous body, ending finally 
in a point at S, that of the half shadow increases more and more with this 
distance. fig. 3 represents the appearance which would be presented by 
their shadows, if received at m’n, on a screen. It will be seen that the 
central shadow is smaller, and the half shadow larger, with the distance 
from the body producing the shadow, until the former vanishes entirely, and 
only the latter remains. This increases in size, but at the same time 
diminishes in intensity until it also disappears. 
If the light from a luminous or illuminated body falls upon a screen with a 
small opening, the light passing through forms a well defined beam, 
producing upon a second screen a bright spot on a dark ground. If an 
aperture of this character be made in the window shutter of a perfectly dark 
room, an inverted image of external objects will be found upon the opposite 
wall (fig. 4). A beam of solar light under such circumstances presents a 
round image, even though the aperture be angular, as a circular image Is 
formed by every point of the aperture, and the combination of these innu- 
merable round images must necessarily give a single image that is 
round. 
The velocity of light is extraordinarily great. It passes from the sun to 
the earth in eight minutes and thirty-six seconds, and in each second 
traverses not less than 192,000 miles. It has been a problem in Astronomy 
to determine this velocity by observations on the motions of Jupiter’s 
satellites (see page 116). The calculations were first made by Olaus 
Romer and Cassini. 
b, Reflection of Light.—Catoptrics. 
When a ray of light strikes a very smooth level surface, a polished 
glass or metallic plate for instance, it is reflected, and the angle formed by 
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