Of the houses of a town, in which the divisions between the houses
may be distinguished by the light which fall on the mist at the
bottom. If the eye is above the houses the light seen in the space
that is between one house and the next sinks by degrees into thicker
mist; and yet, being less transparent, it appears whiter; and if the
houses are some higher than the others, since the true [colour] is
always more discernible through the thinner atmosphere, the houses
will look darker in proportion as they are higher up. Let _n o p q_
represent the various density of the atmosphere thick with moisture,
_a_ being the eye, the house _b c_ will look lightest at the bottom,
because it is in a thicker atmosphere; the lines _c d f_ will appear
equally light, for although _f_ is more distant than _c_, it is
raised into a thinner atmosphere, if the houses _b e_ are of the
same height, because they cross a brightness which is varied by
mist, but this is only because the line of the eye which starts from
above ends by piercing a lower and denser atmosphere at _d_ than at
_b_. Thus the line a _f_ is lower at _f_ than at _c_; and the house
_f_ will be seen darker at _e_ from the line _e k_ as far as _m_,
than the tops of the houses standing in front of it.