VINEGAR. See "Acetic Acid." VisroN. (Latin, videre to see.) The phenomena of human vision voill be described under two heads, viz. monocular vision, and binocular vision.
Monocular Vision.
Vision is said to be " monocular" when only one eye is employed.
The human eye may be considered as a sort of spherical camera obscura, in which the pupil, (or little black dot in the centre of the eye), is the diaphragm, and the retina the focussing screen ; this admirable natural camera being then placed within a socket lined with fat, in which it works by means of voluntary muscles that are attached to it, and by which its axis is directed. towards any point with astonishing rapidity and precision.
The following account of the human eye is so excellent, that we extract it verbatim from a Treatise on Optics published by the Society for the Diffusion of Useful Knowledge.
" The human eye, of which a vertical section is given in the follow ing figure, is nearly of a globular form, with a slight elongation or projection in front. It con sises of- four coats or membranes, viz. the Sclerotic, the Cornea, the Choroid, and the _Retina ; of two fluids or humours, the Aqueous and the Vitreous ; and of one lens, called the Crystal line. The Selerotic coat, a a a, is the outer and strongest coat, to which the muscles for giving it motion are attached. It con stitutes the white of the eye. It is joined to the Cornea, b b, or the clear and transparent circular membrane through which we see. The cornea, which is equally thick throughout, is very tough, and consists of several layers or folds to give it strength, so as to defend the delicate parts within from external injury. On the inner surface of the sclerotic coat is a delicate membiane, called the Clioroid coat, which is covered with a black pigment. Onthe inner side of this lies the Retina, rrrr, which is the innermost coat, and is a tender reticular membrane, formed from the expansion of the optic nerve, which enters the eye at 0, a little more than one-tenth of an inch from the axis on the side towards the nose. At the end of the axis of the eye, and in the very centre of the retina, there is a small hole, with a yellow margin. It is called the foramen centrale, or central hole, though it is not a hole but merely a transparent spot, free of the soft pulpy matter of which the retina consists.
" A flat membrane of a circular form, e f, called the iris, and seen through the cornea b b, divides the interior globe of the eye into two very unequal parts. It has a circular opening, in its centre, called
the pupil, which expands when the light which enters the eye is diminished, and contracts when the light is increased. The space before the iris, called the anterior chamber of the eye, contains the aqueous humour, from its resemblance to pure water ; and the space behind the iris is called' the posterior chamber, and contains the crys talline lens, c c, and the vitreous humour, which fills all the rest of the eye. The crystalline lens is suspended in a transparent capsule, or bag, by what are called the ciliary processes, g g. This lens is more convex behind than in front, as the figure shows ; and it con sists of concentric coats composed of fibres. It increases in density from its circumference to its centre, for the purpose of correcting its spherical aberration. The vitreous humour, ,V V, occupying the largest portion of the eye, lies immediately behind the crystalline lens, and fills the whole space between it and the retina, r r r r.
" The following are the dimensions of the eye, as given by Dr. Young and M. Petit : "Dr. Brewster and Dr. Gordon took the following measures of the crystalline and cornea from the eye of a woman above fifty years of age, a few hours after death.
Diameter of the crystalline . 0'378 Diameter of the cornea . 0'400 Thickness of the crystalline . 0'172 Thickness of the c,ornea . . 0'042 " The following are the refractive powers of the humours of the eye, according to different observers : Aqueous Crystalline Lens Vitreous Humour. Outer Coat Centre Mean Humour Hauksbee . 113395 P33595 Jurin . . . . 11333 Rochon . . . l'329 1'332 Young . . . 11333 Brewster . . 11386 1'3767 1'3990 1'3839 11394 " From the last of these measures we may deduce the following in dices of refraction : Index of Refraction. For rays passing from the aqueous humour into the outer coat of the crystalline lens . 1'04138 For rays passing from the aqueous humour into the crystal line, taking its mean index of refraction . . . 11)353 For rays passing from the outer coat of the crystalline into the vitreous humour . . 0'93 " From the dimensions of the eye given above, and by means of the precedintr indices of refraction, it will be easy to trace, by the method ;heady described, the progress of rays through the humours of the eye, whether they fall upon it in a parallel or a diverging con dition.