[Footnote: 70. 15--23. This section has already been published in the "_Saggio delle Opere di Leonardo da Vinci_" Milan 1872, pp. 13, 14. G. Govi observes upon it, that Leonardo is not to be regarded as the inventor of the Camera obscura, but that he was the first to explain by it the structure of the eye. An account of the Camera obscura first occurs in CESARE CESARINI's Italian version of Vitruvius, pub. 1523, four years after Leonardo's death. Cesarini expressly names Benedettino Don Papnutio as the inventor of the Camera obscura. In his explanation of the function of the eye by a comparison with the Camera obscura Leonardo was the precursor of G. CARDANO, Professor of Medicine at Bologna (died 1576) and it appears highly probable that this is, in fact, the very discovery which Leonardo ascribes to himself in section 21 without giving any further details.] 71. HOW THE IMAGES OF OBJECTS RECEIVED BY THE EYE INTERSECT WITHIN THE CRYSTALLINE HUMOUR OF THE EYE. An experiment, showing how objects transmit their images or pictures, intersecting within the eye in the crystalline humour, is seen when by some small round hole penetrate the images of illuminated objects into a very dark chamber. Then, receive these images on a white paper placed within this dark room and rather near to the hole and you will see all the objects on the paper in their proper forms and colours, but much smaller; and they will be upside down by reason of that very intersection. These images being transmitted from a place illuminated by the sun will seem actually painted on this paper which must be extremely thin and looked at from behind. And let the little perforation be made in a very thin plate of iron. Let _a b e d e_ be the object illuminated by the sun and _o r_ the front of the dark chamber in which is the said hole at _n m_. Let _s t_ be the sheet of paper intercepting the rays of the images of these objects upside down, because the rays being straight, _a_ on the right hand becomes _k_ on the left, and _e_ on the left becomes _f_ on the right; and the same takes place inside the pupil. [Footnote: This chapter is already known through a translation into French by VENTURI. Compare his '_Essai sur les ouvrages physico-mathématiques de L. da Vinci avec des fragments tirés de ses Manuscrits, apportés de l'Italie. Lu a la premiere classe de l'Institut national des Sciences et Arts.' Paris, An V_ (1797).] The practice of perspective (72. 73). 72. In the practice of perspective the same rules apply to light and to the eye. 73. The object which is opposite to the pupil of the eye is seen by that pupil and that which is opposite to the eye is seen by the pupil. Refraction of the rays falling upon the eye (74. 75) 74. The lines sent forth by the image of an object to the eye do not reach the point within the eye in straight lines. 75.