ELEMENTARY EXPERIMENTAL PHYSIOLOGY 255 



various movements of the eyes should occur, so that the two images 

 should fall on corresponding points. 



With reference to the movements of the eyes, it is customary to 

 regard them as taking place about three axes : (a) the sagittal axis, 

 corresponding nearly to the line of sight; (b) the frontal axis, extending 

 from right to left in each eye ; and (c) the vertical axis. These axes are 

 regarded as intersecting at one point the centre of rotation of the eye. 

 With the head in fixed position the extent of space in which objects can 

 be seen by allowing the maximum of eye movement is called the Field 

 of Regard. If the head and body are erect and the eyes are directed 

 towards the distant horizon, the position assumed is spoken of as the 

 Primary Position. The point upon which the eyes are fixed is called 

 the Principal Point of Regard. A position which the eyes may take up 

 which does not conform to the requirements of the Primary Position is 

 called a Secondary Position. If an observer shift his gaze from the 

 principal point of regard to some other point in the field of regard, he 

 may pass directly to this new position, or may pass over a varied 

 number of different points in the field of regard before reaching this 

 final position. The amount of rotation about the different axes of the 

 eye finally involved in adopting this new position will be the same 

 whether the eye pass to it directly or by a number of varied inter- 

 mediate positions. In other words, only one position is possible when 

 the gaze is shifted to this second point. This is called Donders' 

 law. An extension of the rule is seen in Listing's law, which lays 

 down that in moving from the primary position there is no rotation 

 at all upon the sagittal axis, but merely upon the horizontal and 

 vertical axes. 



1. Binocular Direction. In judging of the position of near objects, 

 they are referred not to either eye separately, but to an ideal eye 

 situated midway between the two actual eyes, the so-called Cyclopean 

 eye of Hering. A line drawn through the object to the centre of such 

 an eye is the Binocular Line of Regard. 



EXPERIMENT. Make a pinhole in a sheet of paper, and starting with 

 the hole well to the right of the right eye, draw the paper across the 

 eye horizontally, so that the pinhole will pass across each eye succes- 

 sively. First one and then a second image of the pinhole will be. seen 

 as it passes over each eye, but in either case the hole will be referred to 

 the median plane or the Cyclopean eye, and will seem like a succession 

 of two holes over this eye. 



2. Single and Double Images. If the two eyes be directed towards 

 an object about two feet off, and a finger be held up in the binocular 



