ACCOMMODATION OF THE EYE. 831 



is the greater the more obliquely the plate is set. Hence, if the observer A looks 

 through the telescope F, in front of whose objective (in its upper half) the oblique 

 plate G' is placed, and sees the corneal image ab of the eye B, the latter appears 

 to be displaced laterally, that is to a' b r . If a second plate G is placed before the 

 lower half of the eye-piece of the telescope, and is inclined in an opposite direction 

 (so that both plates meet at an angle, in the horizontal diameter of the objective) 

 the observer sees the corneal image a b displaced laterally to a" b". As the two 

 glass plates may be rotated on each other (at their points of intersection) they are 

 so placed that the two reflected images have their inner edges exactly in contact 

 (so that b' touches a"). From the size of the angle that the two plates make, 

 the size of the image may be calculated (but the thickness of the glass plates, and 

 the refractive index of the glass must be taken into consideration). In this 

 way the size of the reflected image of the cornea and also of the lens can be deter- 

 mined, both in the state of rest, and in that of accommodation for near vision. 



All of the ocular media, including the retina, have a certain amount of fluores- 

 cence, the lens most, the vitreous least. 



As the retinal image is inverted, the perception of the object as an 

 erect one is yet to be explained. By a psychical act the impulses from 

 each point of the retina are projected outward: thus the stimulation of 

 the point d (Fig. 279) to A, that of c to B. This projection outward 

 is so accomplished that all the points appear to lie in a surface suspended 

 before the eye, which is called the "field of vision." This field of 

 vision is therefore the surface of the retina projected outward and in- 

 verted; consequently the field of vision appears erect, as the inverted 

 retinal image is projected outward and inverted. 



That the stimulation of each point is projected in an inverse direction through 

 the nodal point is shown by the simple experiment that pressure on the outer 

 side of the eyeball is referred to the inner aspect of the visual field. The entoptical 

 phenomena of the retina are likewise projected outward and inverted; so that, 

 for example, the point of entrance of the optic nerve is referred to the outer side 

 of the yellow spot, and the like. All sensations of the retina are thus referred 

 externally. ,,Wir sehen die Sonne, die Sterne an den Himmel, nicht an dem Him- 

 rnel" (v. Helmholtz). 



ACCOMMODATION OF THE EYE. 



The image of a point of light, as, for example, of a flame, formed by a convex 

 lens, is always at a definite distance from the point (according to rule 2, page 

 824). If a projecti on -surf ace (a screen) is placed in this position, a real and 

 inverted image is formed upon it. If, however, the screen is placed close to the 

 lens (Fig. 272, IV, ab) or farther aw r ay from it (cd), the image is not distinct, and 

 diffusion-circles are formed: in the first instance, because the rays have not yet 

 united; in the second, because the rays have already crossed and are again 

 diverging. When the point of light is alternately approached to and withdrawn 

 from the lens, the screen must be correspondingly moved closer to the lens or 

 farther away from it, if the sharpness of the image is to be preserved. If the 

 screen is fixed, while the distance of the point of light from the lens varies, a sharp 

 image could be formed only by increasing the curvature of the lens, and thus 

 increasing its refractive power when the point of light is approached to the lens, 

 or by diminishing its curvature, and thus making it less refractive, when the 

 point of light is withdrawn. 



As the eye has its surface of projection (retina) fixed at an unchanging position, 

 and as the eye possesses the ability to form on the retina sharp images of both 

 far and near objects, it must possess the power of altering its refractive strength 

 (the form of the lens) to correspond in every case to the distance of the object. 



By accommodation is meant the power of the eye to form sharp 

 images of both distant and near objects upon the retina. This depends 

 upon its ability to make the lens more or less convex (thicker or thinner), 

 according to the distance of the object. If the lens is absent, accom- 

 modation is impossible. 



