ACCOMMODATION OF THE EYE. 749 



image appears to be displaced laterally, viz., to a' V. If a second plate, G, be placed in front 

 of the loiver half of the telescope, but placed in the opposite direction, so that both plates, 

 corresponding to the middle line of the objective, intersect at an angle, then the observer sees 

 the reflected image, a b, displaced laterally to a" b". As both glass plates rotate round their 

 point of intersection, the position of both is so selected, that both reflected images just touch 

 each other with their inner margins, (so that b' abuts closely upon a"). The size of the reflected 

 image can be determined from the size of the angle formed by both plates, but we must take 

 into calculation the thickness of the glass plates and their refractive indices. The size of the 

 corneal image, and also that in the lens, may be ascertained in the passive eye, and also in the 

 eye accommodated for a near object, and the length of the radius of the curved surface may be 

 calculated therefrom {Helmholtz and others). 



Fluorescence. All the media of the eye, even the retina, are slightly fluorescent; the lens 

 most, the virteous humour least (v. Helmholtz). 



Erect Vision. As the retinal image is inverted, we must explain how we see 

 objects upright. By a psychical act, the impulses from any point of the retina are 

 again referred to the exterior, in the direction through the nodal point ; thus the 

 stimulation of the point d is referred to A, that of c to B (fig. 531). The reference 

 of the image to the external world happens thus, that all points appear to lie in a 

 surface floating in front of the eye, which is called the field of vision. The field of 

 vision is the inverted surface of the retina projected externally ; hence, the field of 

 vision appears erect again, as the inverted retinal image is again projected externally 

 but inverted (fig. 531). 



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

 nodal point, is proved by the simple experiment, that pressure upon the outer aspect of the 

 eyeball is projected or referred to the inner aspect of the field of vision. The entoptical phe- 

 nomena of the retina are similarly projected externally and inverted ; so that, e.g., the entrance 

 of the optic nerve is referred externally to the yellow spot (see 393). All sensations from the 

 retina are projected externally. 



387. ACCOMMODATION OF THE EYE. According to No. 2. (p. 743), the rays of light 

 proceeding from a luminous point, e.g., a flame, and acted upon by a collecting (convex) lens, 

 are brought to a focus or focal point, which has always a definite relation to the luminous 

 object. If a projection-surface or screen be placed at this distance from the lens, a real and 

 inverted image of the object is obtained upon the screen. If the screen be placed nearer to the 

 lens (fig. 524, IV, a, b), or farther away from it (c, d), no distinct image of the object is formed, 

 but diffusion circles are obtained ; because, in the former case, the rays have not united, and 

 in the latter, because the rays, after uniting, have crossed each other and become divergent. 

 If the luminous point be brought nearer to, or removed farther from, the lens, in order to 

 obtain a distinct image, in every case, the screen must be brought nearer, or removed from the 

 lens, to keep the same distance between the lens and the screen. If, however, the screen be 

 fixed permanently, whilst the distance between the luminous point and the lens varies, a 

 distinct image can only be obtained upon the screen, provided the lens, as the luminous point 

 approaches it, becomes more convex, i.e., refracts the rays of light more strongly conversely, 

 when the distance between the luminous point and the lens becomes greater, the lens must 

 become less curved, i.e., refract less strongly. 



In the eye, the projection surface or screen is represented by the retina, which is permanently 

 fixed at a certain distance ; but the eye has the power of forming distinct images of near and 

 distant objects upon the retina, so that the refractive power, i.e., the form of the crystalline 

 lens in the eye, must undergo a change in curvature corresponding in every case to the distance 

 of the object. [It is important to remember, that we cannot see a near object and a distant 

 one with equal distinctness at the same time, and hence arises the necessity for accom- 

 modation.] 



Accommodation. By the term " accommodation of the eye," is understood that 

 property of the eye, whereby it forms distinct images of distant as well as near 

 objects upon the retina. This power depends upon the fact, that the crystalline 

 lens alters its curvature, becoming more convex (thicker), or less curved 

 (flatter), according to the distance of the object. When the lens is absent from the 

 eyeball, accommodation is impossible (Th. Young, Donders p. 740): 



During rest [or negative accommodation], or when the eye is passive, it is 

 accommodated for the greatest distance, i.e., images of objects placed at an infinite 

 distance (e.g., the moon) are formed upon the retina. In this case, rays coming 



