On the luminosity of the Earth in the universal space (874-878).

874.

In my book I propose to show, how the ocean and the other seas must, by means of the sun, make our world shine with the appearance of a moon, and to the remoter worlds it looks like a star; and this I shall prove.

Show, first that every light at a distance from the eye throws out rays which appear to increase the size of the luminous body; and from this it follows that 2 ...[Footnote 10:  Here the text breaks off; lines 11 and fol. are written in the margin.].

[11]The moon is cold and moist.  Water is cold and moist.  Thus our seas must appear to the moon as the moon does to us.

875.

The waves in water magnify the image of an object reflected in it.

Let a be the sun, and n m the ruffled water, b the image of the sun when the water is smooth.  Let f be the eye which sees the image in all the waves included within the base of the triangle c e f.  Now the sun reflected in the unruffled surface occupied the space c d, while in the ruffled surface it covers all the watery space c e (as is proved in the 4th of my “Perspective”) [Footnote 9:  Nel quarto della mia prospettiva.  If this reference is to the diagrams accompanying the text—­as is usual with Leonardo—­and not to some particular work, the largest of the diagrams here given must be meant.  It is the lowest and actually the fifth, but he would have called it the fourth, for the text here given is preceded on the same page of the manuscript by a passage on whirlpools, with the diagram belonging to it also reproduced here.  The words della mia prospettiva may therefore indicate that the diagram to the preceding chapter treating on a heterogeneal subject is to be excluded.  It is a further difficulty that this diagram belongs properly to lines 9-10 and not to the preceding sentence.  The reflection of the sun in water is also discussed in the Theoretical part of the Book on Painting; see Vol.  I, No. 206, 207.] and it will cover more of the water in proportion as the reflected image is remote from the eye [10].

[Footnote:  In the original sketch, inside the circle in the first diagram, is written Sole (sun), and to the right of it luna (moon).  Thus either of these heavenly bodies may be supposed to fill that space.  Within the lower circle is written simulacro (image).  In the two next diagrams at the spot here marked L the word Luna is written, and in the last sole is written in the top circle at a.]

The image of the sun will be more brightly shown in small waves than in large ones—­and this is because the reflections or images of the sun are more numerous in the small waves than in large ones, and the more numerous reflections of its radiance give a larger light than the fewer.

Waves which intersect like the scales of a fir cone reflect the image of the sun with the greatest splendour; and this is the case because the images are as many as the ridges of the waves on which the sun shines, and the shadows between these waves are small and not very dark; and the radiance of so many reflections together becomes united in the image which is transmitted to the eye, so that these shadows are imperceptible.