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Notebooks of Leonardo Da Vinci, The
VOLUME I   VOLUME I - SECTION I: PROLEGOMENA AND GENERAL INTRODUCTION TO THE BOOK ON PAINTING
Leonardo da Vinci
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       VOLUME I: SECTION I: PROLEGOMENA AND GENERAL INTRODUCTION TO THE BOOK ON PAINTING
       The author's intention to publish his MSS (1)
       1.
       How by a certain machine many may stay some time under water. And
       how and wherefore I do not describe my method of remaining under
       water and how long I can remain without eating. And I do not publish
       nor divulge these, by reason of the evil nature of men, who would
       use them for assassinations at the bottom of the sea by destroying
       ships, and sinking them, together with the men in them. Nevertheless
       I will impart others, which are not dangerous because the mouth of
       the tube through which you breathe is above the water, supported on
       air sacks or cork.
       [Footnote: The leaf on which this passage is written, is headed with
       the words _Casi_ 39, and most of these cases begin with the word
       '_Come_', like the two here given, which are the 26th and 27th. 7.
       _Sughero_. In the Codex Antlanticus 377a; 1170a there is a sketch,
       drawn with the pen, representing a man with a tube in his mouth, and
       at the farther end of the tube a disk. By the tube the word
       '_Channa_' is written, and by the disk the word '_sughero_'.]
       The preparation of the MSS. for publication (2)
       2.
       When you put together the science of the motions of water, remember
       to include under each proposition its application and use, in order
       that this science may not be useless.--
       [Footnote: A comparatively small portion of Leonardo's notes on
       water-power was published at Bologna in 1828, under the title: "_Del
       moto e misura dell'Acqua, di L. da Vinci_".]
       Admonition to readers (3)
       3.
       Let no man who is not a Mathematician read the elements of my work.
       The disorder in the MSS (4)
       4.
       Begun at Florence, in the house of Piero di Braccio Martelli, on the
       22nd day of March 1508. And this is to be a collection without
       order, taken from many papers which I have copied here, hoping to
       arrange them later each in its place, according to the subjects of
       which they may treat. But I believe that before I am at the end of
       this [task] I shall have to repeat the same things several times;
       for which, O reader! do not blame me, for the subjects are many and
       memory cannot retain them [all] and say: 'I will not write this
       because I wrote it before.' And if I wished to avoid falling into
       this fault, it would be necessary in every case when I wanted to
       copy [a passage] that, not to repeat myself, I should read over all
       that had gone before; and all the more since the intervals are long
       between one time of writing and the next.
       [Footnote: 1. In the history of Florence in the early part of the
       XVIth century _Piero di Braccio Martelli_ is frequently mentioned as
       _Commissario della Signoria_. He was famous for his learning and at
       his death left four books on Mathematics ready for the press; comp.
       LITTA, _Famiglie celebri Italiane_, _Famiglia Martelli di
       Firenze_.--In the Official Catalogue of MSS. in the Brit. Mus., New
       Series Vol. I., where this passage is printed, _Barto_ has been
       wrongly given for Braccio.
       Footnote: 2. _addi 22 di marzo 1508_. The Christian era was computed in
       Florence at that time from the Incarnation (Lady day, March 25th).
       Hence this should be 1509 by our reckoning.
       Footnote: 3. _racolto tratto di molte carte le quali io ho qui copiate_. We
       must suppose that Leonardo means that he has copied out his own MSS.
       and not those of others. The first thirteen leaves of the MS. in the
       Brit. Mus. are a fair copy of some notes on physics.]
       Suggestions for the arrangement of MSS treating of particular subjects (5-8)
       5.
       Of digging a canal. Put this in the Book of useful inventions and in
       proving them bring forward the propositions already proved. And this
       is the proper order; since if you wished to show the usefulness of
       any plan you would be obliged again to devise new machines to prove
       its utility and thus would confuse the order of the forty Books and
       also the order of the diagrams; that is to say you would have to mix
       up practice with theory, which would produce a confused and
       incoherent work.
       6.
       I am not to blame for putting forward, in the course of my work on
       science, any general rule derived from a previous conclusion.
       7.
       The Book of the science of Mechanics must precede the Book of useful
       inventions.--Have your books on anatomy bound! [Footnote: 4. The
       numerous notes on anatomy written on loose leaves and now in the
       Royal collection at Windsor can best be classified in four Books,
       corresponding to the different character and size of the paper. When
       Leonardo speaks of '_li tua libri di notomia_', he probably means
       the MSS. which still exist; if this hypothesis is correct the
       present condition of these leaves might seem to prove that he only
       carried out his purpose with one of the Books on anatomy. A borrowed
       book on Anatomy is mentioned in F.O.]
       8.
       The order of your book must proceed on this plan: first simple
       beams, then (those) supported from below, then suspended in part,
       then wholly [suspended]. Then beams as supporting other weights
       [Footnote: 4. Leonardo's notes on Mechanics are extraordinarily
       numerous; but, for the reasons assigned in my introduction, they
       have not been included in the present work.].
       General introductions to the book on Painting (9-13)
       9.
       INTRODUCTION.
       Seeing that I can find no subject specially useful or
       pleasing--since the men who have come before me have taken for their
       own every useful or necessary theme--I must do like one who, being
       poor, comes last to the fair, and can find no other way of providing
       himself than by taking all the things already seen by other buyers,
       and not taken but refused by reason of their lesser value. I, then,
       will load my humble pack with this despised and rejected
       merchandise, the refuse of so many buyers; and will go about to
       distribute it, not indeed in great cities, but in the poorer towns,
       taking such a price as the wares I offer may be worth. [Footnote: It
       need hardly be pointed out that there is in this 'Proemio' a covert
       irony. In the second and third prefaces, Leonardo characterises his
       rivals and opponents more closely. His protest is directed against
       Neo-latinism as professed by most of the humanists of his time; its
       futility is now no longer questioned.]
       10.
       INTRODUCTION.
       I know that many will call this useless work [Footnote: 3. questa
       essere opera inutile. By opera we must here understand libro di
       pittura and particularly the treatise on Perspective.]; and they
       will be those of whom Demetrius [Footnote: 4. Demetrio. "With regard
       to the passage attributed to Demetrius", Dr. H. MULLER STRUBING
       writes, "I know not what to make of it. It is certainly not
       Demetrius Phalereus that is meant and it can hardly be Demetrius
       Poliorcetes. Who then can it be--for the name is a very common one?
       It may be a clerical error for Demades and the maxim is quite in the
       spirit of his writings I have not however been able to find any
       corresponding passage either in the 'Fragments' (C. MULLER, _Orat.
       Att._, II. 441) nor in the Supplements collected by DIETZ (_Rhein.
       Mus._, vol. 29, p. 108)."
       The same passage occurs as a simple Memorandum in the MS. Tr. 57,
       apparently as a note for this '_Proemio_' thus affording some data
       as to the time where these introductions were written.] declared
       that he took no more account of the wind that came out their mouth
       in words, than of that they expelled from their lower parts: men who
       desire nothing but material riches and are absolutely devoid of that
       of wisdom, which is the food and the only true riches of the mind.
       For so much more worthy as the soul is than the body, so much more
       noble are the possessions of the soul than those of the body. And
       often, when I see one of these men take this work in his hand, I
       wonder that he does not put it to his nose, like a monkey, or ask me
       if it is something good to eat.
       [Footnote: In the original, the Proemio di prospettiva cioe
       dell'uffitio dell'occhio (see No. 21) stands between this and the
       preceding one, No. 9.]
       INTRODUCTION.
       I am fully concious that, not being a literary man, certain
       presumptuous persons will think that they may reasonably blame me;
       alleging that I am not a man of letters. Foolish folks! do they not
       know that I might retort as Marius did to the Roman Patricians
       [Footnote 21: _Come Mario disse ai patriti Romani_. "I am unable to
       find the words here attributed by Leonardo to Marius, either in
       Plutarch's Life of Marius or in the Apophthegmata (_Moralia_,
       p.202). Nor do they occur in the writings of Valerius Maximus (who
       frequently mentions Marius) nor in Velleius Paterculus (II, 11 to
       43), Dio Cassius, Aulus Gellius, or Macrobius. Professor E.
       MENDELSON of Dorpat, the editor of Herodian, assures me that no such
       passage is the found in that author" (communication from Dr. MULLER
       STRUBING). Leonardo evidently meant to allude to some well known
       incident in Roman history and the mention of Marius is the result
       probably of some confusion. We may perhaps read, for Marius,
       Menenius Agrippa, though in that case it is true we must alter
       Patriti to Plebei. The change is a serious one. but it would render
       the passage perfectly clear.] by saying: That they, who deck
       themselves out in the labours of others will not allow me my own.
       They will say that I, having no literary skill, cannot properly
       express that which I desire to treat of [Footnote 26: _le mie cose
       .... che d'altra parola_. This can hardly be reconciled with Mons.
       RAVAISSON'S estimate of L. da Vinci's learning. "_Leonard de Vinci
       etait un admirateur et un disciple des anciens, aussi bien dans
       l'art que dans la science et il tenait a passer pour tel meme aux
       yeux de la posterite._" _Gaz. des Beaux arts. Oct. 1877.]; but they
       do not know that my subjects are to be dealt with by experience
       rather than by words [Footnote 28: See Footnote 26]; and
       [experience] has been the mistress of those who wrote well. And so,
       as mistress, I will cite her in all cases.
       11.
       Though I may not, like them, be able to quote other authors, I shall
       rely on that which is much greater and more worthy:--on experience,
       the mistress of their Masters. They go about puffed up and pompous,
       dressed and decorated with [the fruits], not of their own labours,
       but of those of others. And they will not allow me my own. They will
       scorn me as an inventor; but how much more might they--who are not
       inventors but vaunters and declaimers of the works of others--be
       blamed.
       INTRODUCTION.
       And those men who are inventors and interpreters between Nature and
       Man, as compared with boasters and declaimers of the works of
       others, must be regarded and not otherwise esteemed than as the
       object in front of a mirror, when compared with its image seen in
       the mirror. For the first is something in itself, and the other
       nothingness.--Folks little indebted to Nature, since it is only by
       chance that they wear the human form and without it I might class
       them with the herds of beasts.
       12.
       Many will think they may reasonably blame me by alleging that my
       proofs are opposed to the authority of certain men held in the
       highest reverence by their inexperienced judgments; not considering
       that my works are the issue of pure and simple experience, who is
       the one true mistress. These rules are sufficient to enable you to
       know the true from the false--and this aids men to look only for
       things that are possible and with due moderation--and not to wrap
       yourself in ignorance, a thing which can have no good result, so
       that in despair you would give yourself up to melancholy.
       13.
       Among all the studies of natural causes and reasons Light chiefly
       delights the beholder; and among the great features of Mathematics
       the certainty of its demonstrations is what preeminently (tends to)
       elevate the mind of the investigator. Perspective, therefore, must
       be preferred to all the discourses and systems of human learning. In
       this branch [of science] the beam of light is explained on those
       methods of demonstration which form the glory not so much of
       Mathematics as of Physics and are graced with the flowers of both
       [Footnote: 5. Such of Leonardo's notes on Optics or on Perspective
       as bear exclusively on Mathematics or Physics could not be included
       in the arrangement of the _libro di pittura_ which is here presented
       to the reader. They are however but few.]. But its axioms being laid
       down at great length, I shall abridge them to a conclusive brevity,
       arranging them on the method both of their natural order and of
       mathematical demonstration; sometimes by deduction of the effects
       from the causes, and sometimes arguing the causes from the effects;
       adding also to my own conclusions some which, though not included in
       them, may nevertheless be inferred from them. Thus, if the Lord--who
       is the light of all things--vouchsafe to enlighten me, I will treat
       of Light; wherefore I will divide the present work into 3 Parts
       [Footnote: 10. In the middle ages--for instance, by ROGER BACON, by
       VITELLONE, with whose works Leonardo was certainly familiar, and by
       all the writers of the Renaissance Perspective and Optics were not
       regarded as distinct sciences. Perspective, indeed, is in its widest
       application the science of seeing. Although to Leonardo the two
       sciences were clearly separate, it is not so as to their names; thus
       we find axioms in Optics under the heading Perspective. According to
       this arrangement of the materials for the theoretical portion of the
       _libro di pittura_ propositions in Perspective and in Optics stand
       side by side or occur alternately. Although this particular chapter
       deals only with Optics, it is not improbable that the words _partiro
       la presente opera in 3 parti_ may refer to the same division into
       three sections which is spoken of in chapters 14 to 17.].
       The plan of the book on Painting (14-17)
       14.
       ON THE THREE BRANCHES OF PERSPECTIVE.
       There are three branches of perspective; the first deals with the
       reasons of the (apparent) diminution of objects as they recede from
       the eye, and is known as Diminishing Perspective.--The second
       contains the way in which colours vary as they recede from the eye.
       The third and last is concerned with the explanation of how the
       objects [in a picture] ought to be less finished in proportion as
       they are remote (and the names are as follows):
       Linear Perspective. The Perspective of Colour. The Perspective of
       Disappearance.
       [Footnote: 13. From the character of the handwriting I infer that
       this passage was written before the year 1490.].
       15.
       ON PAINTING AND PERSPECTIVE.
       The divisions of Perspective are 3, as used in drawing; of these,
       the first includes the diminution in size of opaque objects; the
       second treats of the diminution and loss of outline in such opaque
       objects; the third, of the diminution and loss of colour at long
       distances.
       [Footnote: The division is here the same as in the previous chapter
       No. 14, and this is worthy of note when we connect it with the fact
       that a space of about 20 years must have intervened between the
       writing of the two passages.]
       16.
       THE DISCOURSE ON PAINTING.
       Perspective, as bearing on drawing, is divided into three principal
       sections; of which the first treats of the diminution in the size of
       bodies at different distances. The second part is that which treats
       of the diminution in colour in these objects. The third [deals with]
       the diminished distinctness of the forms and outlines displayed by
       the objects at various distances.
       17.
       ON THE SECTIONS OF [THE BOOK ON] PAINTING.
       The first thing in painting is that the objects it represents should
       appear in relief, and that the grounds surrounding them at different
       distances shall appear within the vertical plane of the foreground
       of the picture by means of the 3 branches of Perspective, which are:
       the diminution in the distinctness of the forms of the objects, the
       diminution in their magnitude; and the diminution in their colour.
       And of these 3 classes of Perspective the first results from [the
       structure of] the eye, while the other two are caused by the
       atmosphere which intervenes between the eye and the objects seen by
       it. The second essential in painting is appropriate action and a due
       variety in the figures, so that the men may not all look like
       brothers, &c.
       [Footnote: This and the two foregoing chapters must have been
       written in 1513 to 1516. They undoubtedly indicate the scheme which
       Leonardo wished to carry out in arranging his researches on
       Perspective as applied to Painting. This is important because it is
       an evidence against the supposition of H. LUDWIG and others, that
       Leonardo had collected his principles of Perspective in one book so
       early as before 1500; a Book which, according to the hypothesis,
       must have been lost at a very early period, or destroyed possibly,
       by the French (!) in 1500 (see H. LUDWIG. L. da Vinci: _Das Buch van
       der Malerei_. Vienna 1882 III, 7 and 8).]
       The use of the book on Painting (18)
       18.
       These rules are of use only in correcting the figures; since every
       man makes some mistakes in his first compositions and he who knows
       them not, cannot amend them. But you, knowing your errors, will
       correct your works and where you find mistakes amend them, and
       remember never to fall into them again. But if you try to apply
       these rules in composition you will never make an end, and will
       produce confusion in your works.
       These rules will enable you to have a free and sound judgment; since
       good judgment is born of clear understanding, and a clear
       understanding comes of reasons derived from sound rules, and sound
       rules are the issue of sound experience--the common mother of all
       the sciences and arts. Hence, bearing in mind the precepts of my
       rules, you will be able, merely by your amended judgment, to
       criticise and recognise every thing that is out of proportion in a
       work, whether in the perspective or in the figures or any thing
       else.
       Necessity of theoretical knowledge (19-20)
       19.
       OF THE MISTAKES MADE BY THOSE WHO PRACTISE WITHOUT KNOWLEDGE.
       Those who are in love with practice without knowledge are like the
       sailor who gets into a ship without rudder or compass and who never
       can be certain whether he is going. Practice must always be founded
       on sound theory, and to this Perspective is the guide and the
       gateway; and without this nothing can be done well in the matter of
       drawing.
       20.
       The painter who draws merely by practice and by eye, without any
       reason, is like a mirror which copies every thing placed in front of
       it without being conscious of their existence.
       The function of the eye (21-23)
       21.
       INTRODUCTION TO PERSPECTIVE:--THAT IS OF THE FUNCTION OF THE EYE.
       Behold here O reader! a thing concerning which we cannot trust our
       forefathers, the ancients, who tried to define what the Soul and
       Life are--which are beyond proof, whereas those things, which can at
       any time be clearly known and proved by experience, remained for
       many ages unknown or falsely understood. The eye, whose function we
       so certainly know by experience, has, down to my own time, been
       defined by an infinite number of authors as one thing; but I find,
       by experience, that it is quite another. [Footnote 13: Compare the
       note to No. 70.]
       [Footnote: In section 13 we already find it indicated that the study
       of Perspective and of Optics is to be based on that of the functions
       of the eye. Leonardo also refers to the science of the eye, in his
       astronomical researches, for instance in MS. F 25b '_Ordine del
       provare la terra essere una stella: Imprima difinisce l'occhio'_,
       &c. Compare also MS. E 15b and F 60b. The principles of astronomical
       perspective.]
       22.
       Here [in the eye] forms, here colours, here the character of every
       part of the universe are concentrated to a point; and that point is
       so marvellous a thing ... Oh! marvellous, O stupendous Necessity--by
       thy laws thou dost compel every effect to be the direct result of
       its cause, by the shortest path. These [indeed] are miracles;...
       In so small a space it can be reproduced and rearranged in its whole
       expanse. Describe in your anatomy what proportion there is between
       the diameters of all the images in the eye and the distance from
       them of the crystalline lens.
       23.
       OF THE 10 ATTRIBUTES OF THE EYE, ALL CONCERNED IN PAINTING.
       Painting is concerned with all the 10 attributes of sight; which
       are:--Darkness, Light, Solidity and Colour, Form and Position,
       Distance and Propinquity, Motion and Rest. This little work of mine
       will be a tissue [of the studies] of these attributes, reminding the
       painter of the rules and methods by which he should use his art to
       imitate all the works of Nature which adorn the world.
       Variability of the eye (24)
       24
       ON PAINTING.
       1st. The pupil of the eye contracts, in proportion to the increase
       of light which is reflected in it. 2nd. The pupil of the eye expands
       in proportion to the diminution in the day light, or any other
       light, that is reflected in it. 3rd. [Footnote: 8. The subject of
       this third proposition we find fully discussed in MS. G. 44a.]. The
       eye perceives and recognises the objects of its vision with greater
       intensity in proportion as the pupil is more widely dilated; and
       this can be proved by the case of nocturnal animals, such as cats,
       and certain birds--as the owl and others--in which the pupil varies
       in a high degree from large to small, &c., when in the dark or in
       the light. 4th. The eye [out of doors] in an illuminated atmosphere
       sees darkness behind the windows of houses which [nevertheless] are
       light. 5th. All colours when placed in the shade appear of an equal
       degree of darkness, among themselves. 6th. But all colours when
       placed in a full light, never vary from their true and essential
       hue.
       Focus of sight(25)
       25.
       OF THE EYE.
       If the eye is required to look at an object placed too near to it,
       it cannot judge of it well--as happens to a man who tries to see the
       tip of his nose. Hence, as a general rule, Nature teaches us that an
       object can never be seen perfectly unless the space between it and
       the eye is equal, at least, to the length of the face.
       Differences of perception by one eye and by both eyes (26-29)
       26.
       OF THE EYE.
       When both eyes direct the pyramid of sight to an object, that object
       becomes clearly seen and comprehended by the eyes.
       27.
       Objects seen by one and the same eye appear sometimes large, and
       sometimes small.
       28.
       The motion of a spectator who sees an object at rest often makes it
       seem as though the object at rest had acquired the motion of the
       moving body, while the moving person appears to be at rest.
       ON PAINTING.
       Objects in relief, when seen from a short distance with one eye,
       look like a perfect picture. If you look with the eye _a_, _b_ at
       the spot _c_, this point _c_ will appear to be at _d_, _f_, and if
       you look at it with the eye _g_, _h_ will appear to be at _m_. A
       picture can never contain in itself both aspects.
       29.
       Let the object in relief _t_ be seen by both eyes; if you will look
       at the object with the right eye _m_, keeping the left eye _n_ shut,
       the object will appear, or fill up the space, at _a_; and if you
       shut the right eye and open the left, the object (will occupy the)
       space _b_; and if you open both eyes, the object will no longer
       appear at _a_ or _b_, but at _e_, _r_, _f_. Why will not a picture
       seen by both eyes produce the effect of relief, as [real] relief
       does when seen by both eyes; and why should a picture seen with one
       eye give the same effect of relief as real relief would under the
       same conditions of light and shade?
       [Footnote: In the sketch, _m_ is the left eye and _n_ the right,
       while the text reverses this lettering. We must therefore suppose
       that the face in which the eyes _m_ and _n_ are placed is opposite
       to the spectator.]
       The comparative size of the image depends on the amount of light (30-39)
       30.
       The eye will hold and retain in itself the image of a luminous body
       better than that of a shaded object. The reason is that the eye is
       in itself perfectly dark and since two things that are alike cannot
       be distinguished, therefore the night, and other dark objects cannot
       be seen or recognised by the eye. Light is totally contrary and
       gives more distinctness, and counteracts and differs from the usual
       darkness of the eye, hence it leaves the impression of its image.
       31.
       Every object we see will appear larger at midnight than at midday,
       and larger in the morning than at midday.
       This happens because the pupil of the eye is much smaller at midday
       than at any other time.
       32.
       The pupil which is largest will see objects the largest. This is
       evident when we look at luminous bodies, and particularly at those
       in the sky. When the eye comes out of darkness and suddenly looks up
       at these bodies, they at first appear larger and then diminish; and
       if you were to look at those bodies through a small opening, you
       would see them smaller still, because a smaller part of the pupil
       would exercise its function.
       [Footnote: 9. _buso_ in the Lomb. dialect is the same as _buco_.]
       33.
       When the eye, coming out of darkness suddenly sees a luminous body,
       it will appear much larger at first sight than after long looking at
       it. The illuminated object will look larger and more brilliant, when
       seen with two eyes than with only one. A luminous object will appear
       smaller in size, when the eye sees it through a smaller opening. A
       luminous body of an oval form will appear rounder in proportion as
       it is farther from the eye.
       34.
       Why when the eye has just seen the light, does the half light look
       dark to it, and in the same way if it turns from the darkness the
       half light look very bright?
       35.
       ON PAINTING.
       If the eye, when [out of doors] in the luminous atmosphere, sees a
       place in shadow, this will look very much darker than it really is.
       This happens only because the eye when out in the air contracts the
       pupil in proportion as the atmosphere reflected in it is more
       luminous. And the more the pupil contracts, the less luminous do the
       objects appear that it sees. But as soon as the eye enters into a
       shady place the darkness of the shadow suddenly seems to diminish.
       This occurs because the greater the darkness into which the pupil
       goes the more its size increases, and this increase makes the
       darkness seem less.
       [Footnote 14: _La luce entrera_. _Luce_ occurs here in the sense of
       pupil of the eye as in no 51: C. A. 84b; 245a; I--5; and in many
       other places.]
       36.
       ON PERSPECTIVE.
       The eye which turns from a white object in the light of the sun and
       goes into a less fully lighted place will see everything as dark.
       And this happens either because the pupils of the eyes which have
       rested on this brilliantly lighted white object have contracted so
       much that, given at first a certain extent of surface, they will
       have lost more than 3/4 of their size; and, lacking in size, they
       are also deficient in [seeing] power. Though you might say to me: A
       little bird (then) coming down would see comparatively little, and
       from the smallness of his pupils the white might seem black! To this
       I should reply that here we must have regard to the proportion of
       the mass of that portion of the brain which is given up to the sense
       of sight and to nothing else. Or--to return--this pupil in Man
       dilates and contracts according to the brightness or darkness of
       (surrounding) objects; and since it takes some time to dilate and
       contract, it cannot see immediately on going out of the light and
       into the shade, nor, in the same way, out of the shade into the
       light, and this very thing has already deceived me in painting an
       eye, and from that I learnt it.
       37.
       Experiment [showing] the dilatation and contraction of the pupil,
       from the motion of the sun and other luminaries. In proportion as
       the sky is darker the stars appear of larger size, and if you were
       to light up the medium these stars would look smaller; and this
       difference arises solely from the pupil which dilates and contracts
       with the amount of light in the medium which is interposed between
       the eye and the luminous body. Let the experiment be made, by
       placing a candle above your head at the same time that you look at a
       star; then gradually lower the candle till it is on a level with the
       ray that comes from the star to the eye, and then you will see the
       star diminish so much that you will almost lose sight of it.
       [Footnote: No reference is made in the text to the letters on the
       accompanying diagram.]
       38.
       The pupil of the eye, in the open air, changes in size with every
       degree of motion from the sun; and at every degree of its changes
       one and the same object seen by it will appear of a different size;
       although most frequently the relative scale of surrounding objects
       does not allow us to detect these variations in any single object we
       may look at.
       39.
       The eye--which sees all objects reversed--retains the images for
       some time. This conclusion is proved by the results; because, the
       eye having gazed at light retains some impression of it. After
       looking (at it) there remain in the eye images of intense
       brightness, that make any less brilliant spot seem dark until the
       eye has lost the last trace of the impression of the stronger light. _