same manner, and if such cannot
be found he should meditate in solitude with himself, and he will not
find more profitable company.

[Sidenote: Perspective & Mathematics]

57.

In the study of natural causes and reasons light affords the greatest
pleasure to the student; among the great facts of mathematics the
certainty of demonstration most signally elevates the mind of the
student. Perspective must therefore be {108} placed at the head of all
human study and discipline, in the field of which the radiant line is
rendered complex by the methods of demonstration; in it resides the
glory of physics as well as of mathematics, and it is adorned with
flowers of both these sciences.

The laws of those sciences which are capable of extensive analysis I
will confine in brief conclusions, and according to the nature of the
material I will interweave mathematical demonstrations, at times
deducing results from causes, and at times tracing causes by results.
I will add to my conclusions some which are not contained in these, but
which can be deduced from them, if the Lord, the Supreme Light,
illuminates me, so that I may treat of light.

[Sidenote: Of the Method of Learning]

58.

When you will have thoroughly mastered perspective and have learnt by
heart the parts and forms of objects, strive when you go about to
observe. Note and consider the circumstances and the actions or men,
as they talk, dispute, laugh or fight together, and not only the
behaviour of the men themselves, but that of the bystanders who
separate them or look on at these things; and make a note of them, in
this way, with slight marks in your little note-book. And you should
always carry this note-book with you, and it should be of coloured
paper, so that what you {109} write may not be rubbed out; but (when it
is used up) change the old for a new one, since these things should not
be rubbed out, but preserved with great care, because such is the
infinity of the forms and circumstances of objects, that the memory is
incapable of retaining them; wherefore keep these sketches as your
guides and masters.

59.

These rules are only to be used in correcting the figures, since every
man makes some mistakes in his first composition, and he who is not
aware of them cannot correct them; but thou being conscious of thine
errors wilt correct thy work and amend errors where thou findest them,
and take care not to fall into them again. But if thou att