1864.] Human Eye in relation to Binocular Vision. 189 



perception of external objects very much, as is well known. One must be 

 very careful to avoid the use of too strong a light in these experiments, 

 because the nervous apparatus of the eye is easily injured by it ; and the 

 * brightness of these coloured strips when illuminated by common day- 

 light is quite sufficient for our present purpose. 



Now you will perceive easily that these ocular spectra are extremely well 

 adapted to ascertain the position of the eye-ball, because they have a fixed 

 connexion with certain parts of the retina itself. If the eyeball could turn 

 about its visual line as an axis, the ocular spectrum would apparently un- 

 dergo the same degree of rotation ; and hence, when we move about the eye, 

 and at last return to the same direction of the visual line, we can recognize 

 easily and accurately whether the eye has returned into the same position 

 as before, or whether the degree of its rotation round the visual line has 

 been altered. Professor Bonders has proved, by using this very delicate 

 test, that the human eye, in its normal state, returns always into the same 

 position when the visual line is brought into the same direction. The 

 position and direction of the eye are to be determined in this case in refer- 

 ence to the head of the observer ; and I beg you to understand always, 

 when I say that the eye or its visual line is moved upwards or downwards, 

 that it is moved either in the direction of the forehead or in that of the 

 cheek ; and when I say it is moved to the left or to the right, you are to 

 understand the left or right side of the head. Therefore, when the head 

 itself is not in its common vertical position, the vertical line here under- 

 stood is not accordant with the line of the plummet. 



Before the researches of Donders, some observers believed they had 

 found a difference in the relative positions of the eye, when the head was 

 brought into different situations. They had used either small brown spots 

 of the iris, or red vessels in the white of the eye, to ascertain the real 

 position of the eyeball; but their apparent results have been shown to be 

 erroneous by the much more trustworthy method of Donders. 



In the first place, therefore, we may state that the position of the eye- 

 ball depends exclusively upon the direction of the visual line in reference 

 to the position of the head of the observer. But now we must ask, what 

 is the law regulating the position of the eye for every direction of its visual 

 line ? In order to define this law, we must first notice that there exists a 

 certain direction of the visual line, which, in relation to the motions of the 

 eye, is distinguished from all other directions of the eye ; and we may call 

 it the central or primary direction of the visual line. This direction is 

 parallel to the median vertical plane of the head ; and it is horizontal when 

 the head of the observer, who is standing, is kept in a convenient erect 

 position to look at distant points of the horizon. How this primary direc- 

 tion of the visual line may be determined practically with greater accuracy 

 we shall see afterwards. All other directions of the visual line we may 

 call secondary directions. 



A plane which passes through the visual line of the eye, I call a meri- 



