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form more than of any other shape; and when 
lighted, whether by the sun or flame, or by 
apertures admitting light, must have two relative 
extremes of light and shadow, two balancing tints, 
the illuminated and did reflected, divided by 3. 
middle tint or the aerial. The effect of illumin- 
ation by flame or aperture, differs from that of 
the sun in this respect; the su11 illuminates with 
parallel rays, which fall over all parts of the 
enlightened side of the subject, While the light 
of a flame or an aperture only strikes directly 
on the nearest pointof the object, producing an 
effect which more or less resembles the illumin- 
ation of the sun in proportion to the distance 
and dimensions of the object. 
" Let us then suppose a ball to be the object 
on which the light falls, in a direction of forty-. 
fwe degrees or the diagonal of a square, and at 
a -right angle from the ball to the place where 
you stand. One half of the ball will appear 
illuminated, and the other dark. This state of 
the two hemispheres constitutes the two masses 
of light and shadow. In the centre of the mass 
of light falls the focus of the illumination in the