PHYSIOLOGY OF VISION INVERTED IMAGE. 237 



eye, and the brilliancy of the organ: other reflected rays perhaps 

 impinge upon the pigmentum nigrum lining the posterior surface of 

 the iris, and are absorbed by it. From the crystalline the ray emerges 

 into a medium possessing less refractive power ; and, therefore, it is 

 deflected from the perpendicular. The shape, however, of the posterior 

 surface of the lens so modifies the perpendiculars, as to occasion such 

 a degree of convergence, that the oblique ray meets the axis at a focus 

 on the retina. (See Figs. 79 and 113.) In this manner two cones are 

 formed ; one having its apex at the radiant point, and its base on the 

 cornea the objective cone; the other having its apex on the retina, 

 and termed the ocular cone. 



These remarks apply chiefly to the cone proceeding in the direc- 

 tion of the axis of the different humours, from a single radiant point. 

 It is easy to understand, that every portion of the object ABC, Fig. 

 113, must be a radiant point, and project so many cones in an analo- 

 gous manner, which, by impinging upon the retina, form a picture of 

 the object upon that expansion, at g b h. It is important, however, 

 to observe, that the rays proceeding from the upper part of the object 

 fall, after refraction, upon the lower part of the retina; and those from 

 the lower part of the object upon the upper ; so that the picture or 

 representation of the object on the retina must be inverted. How the 

 idea of an erect object is excited in the mind will be the subject of after 

 inquiry. 



When rays, as A g and C A, fall obliquely on a lens, and pass 

 through its centre, they suffer refraction at each of its surfaces ; but 

 as the two refractions are equal, and in opposite directions, they may 

 be esteemed to pursue their course in a straight line. The point #, at 

 which these various rays cross, is called the optic centre of the crys- 

 talline. Each of the straight rays proceeding from a radiant point 

 may be assumed as the axis of all the rays proceeding obliquely from 

 the same point; and the common focus must fall on some part of this 

 axis. In this way the object is represented in miniature, and inverted, 

 on the retina. As, however, the oblique ray has to pass through the 

 cornea and aqueous humour, before it impinges on the crystalline, it 

 undergoes considerable deflection ; and consequently it is not accurate 

 to represent it as pursuing a straight course through the different hu- 

 mours on its way to the retina. The main deflection as in the case of 

 the rays D t s, and E t r, Fig. 113 occurs at the entrance of the rays 

 into the cornea. 



That an inverted representation of external objects is formed within 

 the eye is in accordance with sound theory ; and is supported not 

 only by indirect, but by direct experiment. If a double convex lens 

 be fitted into an opening made in the window-shutter of a darkened 

 chamber, luminous cones will proceed from the different objects on the 

 outside of the house, and converge within ; so that if they be received 

 on a sheet of paper, a beautiful and distinct image of the object will 

 be apparent. This is the well-known instrument, the camera obscura, 

 of which the organ of sight may be regarded as a modification. Mak- 

 ing abstraction, indeed, of the cornea, and of the aqueous and vitreous 

 humours, the representation of the eye in Fig. 113, with the object, 



