VISION. 



By W. H. E. Eivers. 



Contents :— The Dioptric System, p. 1026— Accommodation, p. 1033— The Pupil, 

 p. 1042— The Retina and Visual Sensations. — Physiological Retinal Pro- 

 cesses, p. 1045— Visual Sensations, p. 1052— Adaptation and Induction, p. 1056 

 —The Duration of Stimulus and Sensation, p. 1067— The Brightness of Colour 

 Sensations, p. 1076— Mixture, p. 1081— Vision of Different Regions of the 

 Retina, p. 1083— Colour-Blindness, etc., p. 1088— Binocular Phenomena, p. 

 1098— Functions of the Retinal Structures, p. 1099— Theories of Colour-Vision, 

 p. 1105— Binocular Vision, Movements of the Eyes, and Spatial Per- 

 ception, p. 1122— Movements of the Eyes, p. 1129— Spatial Perception, p. 1132. 



THE DIOPTEIC SYSTEM. 



Static and dynamic refraction. — In the first part of this article 

 the refractive apparatus of the eye will he considered, — first in a con- 

 dition of rest, and then as adjusted to vision at different distances and 

 with different luminosities. 



In order to study the formation of the retinal image, and the 

 conditions by which it is modified, it is necessary to simplify the complex 

 optical system of the eye, and reduce it mathematically to a pair of 

 surfaces, — Listing's schematic eye, — or, still further, to a single surface, — 

 Listing's reduced eye. For this reduction certain data are necessary, 

 namely, the radii of curvature of the refractive surfaces, the distances of 

 these surfaces from one another, and the refractive indices of the 

 transparent media. The cornea and the aqueous humour are so nearly 

 of the same refractive index, that the refraction at the posterior 

 corneal surface may be neglected, and this leaves three surfaces needing 

 investigation, namely, the anterior surface of the cornea, and the anterior 

 and posterior surfaces of the lens. 



The ophthalmometer. — The radii of curvature and the distances 

 of the surfaces from one another might he determined on the dead 

 eye, but, owing to the alteration caused by the diminution of the 

 infra-ocular pressure at death, this is not satisfactory. One difficulty 

 in their measurement during life arises from the unavoidable movements 

 of the living eye, but this difficulty has been overcome by the 

 ophthalmometer first devised by Helmholtz, in which the image reflected 

 ti'iiii any of the refractive surfaces of the eye may be measured, and the 

 curvature calculated from its size. 



The principle upon which Helmholtz's instrument depends, is the 

 apparent deviation of an object seen obliquely through a glass plate. 



