Elements of natural philosophy (Volume 2-3) . qual to Ft, or the image is inthe principal focus. It follows also, that the image, whichwill always be virtual for real objects, will be elliptical,erect, and smaller than the object. § 70. If we make / positive, greater than Ft and less same for virtualthan 2 Fn the object will be virtual; the image real, objectserect, and greater than the object. 232 NATURAL PHILOSOPHY. OF THE EYE AND OF VISION. The eye; § 71- The eye is a collection of refractive media which concentrate the waves of light proceeding from everypoint of an external object, on a t
Elements of natural philosophy (Volume 2-3) . qual to Ft, or the image is inthe principal focus. It follows also, that the image, whichwill always be virtual for real objects, will be elliptical,erect, and smaller than the object. § 70. If we make / positive, greater than Ft and less same for virtualthan 2 Fn the object will be virtual; the image real, objectserect, and greater than the object. 232 NATURAL PHILOSOPHY. OF THE EYE AND OF VISION. The eye; § 71- The eye is a collection of refractive media which concentrate the waves of light proceeding from everypoint of an external object, on a tissue of delicate nerv<called the retina, there forming an image, from which,by some process unknown, our perception of the objectarises. These media are contained in a globular en- Four coatings velope composed of four coatings, two of which, very envelop the , refractive media; unequal in extent, make up the external enclosure of theeye, the others serving as lining to the larger of thesetwo. Fig. 42. Graphicrepresentation of the eye;. The cornea; shape of the eye; The shape of the eye is spherical except immediatelyin front, where it projects beyond the spherical form,as indicated at decV\ which represents a section of thehuman eye through the axis by a horizontal plane. Thispart is called the cornea, and is in shape a segment ofan ellipsoid of revolution about its transverse axis whichcoincides with the axis of the eye, and which has tothe conjugate axis, the ratio of 1,3. It is a strong,horny, and delicately transparent coat. Immediately behind the cornea, and in contact with ELEMENTS OF OPTICS. 233 it, is the first refractive medium, called the aqueous Aqueoushumour\ which is found to consist of nearly pure wa-ter, holding a little muriate of soda and gelatine insolution, with a very slight quantity of albumen. Itsrefractive index is found to be very nearly the same asthat of water, viz. : 1,336, and parallel rays having thedirection of the axis of the eye will, in c
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