-V.Mi PHYSIOLOGY 



has reached the fovea. In this way fixation is effected. If the acuity of 

 the fovea is reduced by disease or by working in a bad light, the definition 

 of the image does not sharply improve as the fovea is reached, and therefore 

 the movement of the eyes is not checked until the image has reached the 

 periphery again. But here the degradation of the image calls for the re- 

 verse process, which again causes the image to pass over the fovea. Repeated 

 oscillations of the eyes therefore occur, which are called nystagmus. The 

 condition is met with in the day blind, since cone vision is defective, in 

 persons whose visual acuity has been lowered by working in a dull light, e.g. 

 miners, and in cases of poisoning by tobacco and alcohol. 



THE HOROPTER. Theory shows that, even when fixation is properly 

 effected so that corresponding retinal points occupy the same meridians, 

 images formed on the retinae do not necessarily fall on corresponding points. 

 For this to be the case, it is necessary also that the objects from which those 

 images are formed should occupy certain definite positions in relationship 

 with one another. For example, if an object 10 feet from the eye is fixated, 

 the images of other objects on either side will fall only on corresponding 

 points if these lie on a circle of 5 feet radius, the centre of which lies between 

 the observer and the object fixated. For calculation shows that only then are 

 the images formed on the two retinae the same distance from the centre. 

 The form of the curve which is called the horopter is found to change with 

 the different directions of the gaze. When the gaze is directed to a point 

 on the floor it is stated that the horopter almost corresponds with the plane 

 of the floor. 



MONOCULAR DEPTH PERCEPTION. The perception of depth 

 with the single eye is found to depend on a number of different factors which 

 as a rule operate together : 



1. The apparent size of objects, the dimensions of which are known. 

 Thus the size of a man being approximately known, his distance away is 

 known from the size of the image which is formed on the retina. The further 

 away he is the smaller his image will appear. 



2. The colour of an object being known, the effect of distance in modifying 

 that colour is used in depth perception. Thus trees which, when near, look 

 yellow-green, when seen at a distance through an intervening layer of haze 

 appear blue-green or even blue. This fact is made use of by artists for 

 expressing distance. 



3. The partial obstruction of a distant plane by objects nearer to the 

 observer. 



4. The shadows which one plane casts upon another. 



5. The intensity of the light which is reflected by the object frequently 

 varies with its shape and position. For example, the shape of a solid sphere 

 can be accurately inferred from the distribution of intensity over its face. 



6. By perspective, which may be defined as the geometrical arrangement 

 of lines in the image formed on the retina. Thus the lines of a tennis ccurt 

 seen diagonally from one side are all found to converge to one or other of 

 two points on the horizon. 



