| 168 | 169 | 170 | 171 | 172 |
| 1 | 306 | 613 | 919 | 1225 |
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n m a, therefore x y is three times as dark as z g; x f, by
intersection faces o b and by direct line o n m a, therefore we
must say that the shadow between f x will be four times as dark as
the shadow z g, because it faces four times as much shadow.
Let a b be the side where the primary shadow is, and b c the
primary light, d will be the spot where it is intercepted,f g
the derived shadow and f e the derived light.
And this must be at the beginning of the explanation.
[Footnote: In the original MS. the text of No. 252 precedes the one
given here. In the text of No. 215 there is a blank space of about
four lines between the lines 2 and 3. The diagram given on Pl. VI,
No. 2 is placed between lines 4 and 5. Between lines 5 and 6 there
is another space of about three lines and one line left blank
between lines 8 and 9. The reader will find the meaning of the whole
passage much clearer if he first reads the final lines 11--13.
Compare also line 4 of No. 270.]
On relative proportion of light and shadows (216--221).
2
16.
That part of the surface of a body on which the images [reflection]
from other bodies placed opposite fall at the largest angle will
170
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