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reflects all the rays, the sensations they might separately produce, blend 

 into the sensation of white; if it reflects a few, it appears differently co- 

 loured, according to the rays it repels ; finally, if all be absorbed, the sen- 

 sation of black is produced, which is merely the negation of all colour. 

 A black body is wrapped in utter darkness, and is visible only by the 

 lustre of those that surround it; 5thly, that from every point in the sur- 

 face of a luminous or enlightened body, there issue a multitude of rays, 

 diverging according to their distance, with a proportionate diminution of 

 their effect; so that the rays from each visible point of the body, form a 

 cone, of which the summit is at that point, and the base, the surface of 

 the eye on which they fall. 



CXVI. Sense of Sight. The eyes, the seat of this sense, are so placed, as 

 to command a great extent of objects at once, and are enclosed in two os- 

 seous cavities, known by the name of orbits. The base of these cavities, 

 is forwards, and shaped obliquely outwards; so that their outward side 

 not being so ong as the others, the ball of the eye supported, on that 

 side, only by soft parts, may be directed outwards and take cognizance 

 of objects placed to a side, without it being necessary, at the same time, 

 to turn the head. In proportion as we descend from man in the scale of 

 animated beings, the shape of the base of the orbits becomes more and 

 more oblique ; the eyes cease to be directed forward, in short, the exter- 

 nal side of the socket disappears, and the sight is entirely directed out- 

 ward, and as the physiognomy derives its principal character from the 

 eyes, its expression is absolutely changed. In certain animals very fleet 

 in running, such as the hare, the lateral situation of the organs of vision, 

 prevents the animals from seeing small objects, placed directly before 

 them, hence those animals, when closely pursued, are so easily caught 

 in the snares which are laid for them. 



The organ of sight consists of three essentially distinct parts. The one 

 set intended to protect the eye-ball, to screen it, at times, from the influ- 

 ence of light, and to maintain it in the conditions necessary to the exer- 

 cise of its functions : these parts are the eye-brows, the eye-lids, and the 

 lachrymal apparatus, and they serve as appendages of the organ. The 

 eye-ball itself contains two parts, answering very different purposes ; the 

 one, formed by nearly the whole globe, is a real optical instrument, pla- 

 ced immediately in front of the retina, and destined to produce on the 

 luminous rays those changes which are indispensable, in the mechanism 

 of vision ; the other, formed by the medullary expansion of the optic 

 nerve, is the immediate organ of that function. It is the retina, which 

 alone is affected by the impression of light, .and set in motion by the con- 

 tact of that very subtle fluid. This impression, this motion, this sensa- 

 tion is transmitted to the cerebral organ, by the optic nerve, the expan- 

 sion of which forms the retina. 



CXVII. Of the eye-brows, the eye-lids, and the lachrymal apparatus (Tu- 

 tamina oculi, Haller.) The more or less dark colour of the hairs of the 

 eye-brows, renders that projection very well adapted to diminish the ef- 

 fect of too vivid a light, by absorbing a part of its rays. The eye-brows 

 answer this purpose, the more completely, from being more projecting, 

 and from the darker colours of the hairs which cover them : hence we de- 

 press the eye-brows, by knitting them transversely, in passing from the 

 dark, into a place strongly illuminated*, which causes an uneasy sensation 

 to the organ of sight, Hence, likewise, the custom that prevails with 

 some southern nations, whose eye-brows are shaded by thicker and dark- 



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