1864.] Human Eye in relation to Binocular Vision. 191 



series of my teeth, so that I kept it firmly between my jaws. The impres- 

 sions of the teeth remain indented in the seaUng-wax ; and when I put 

 my teeth afterwards into these impressions, I am sure that the httle board 

 is brought exactly into the same position, relatively to my head and my 

 eyes, as it was before. On the other end of that little board, which is 

 kept horizontally between the teeth, a vertical piece of wood is fastened, 

 on which I fix horizontally a little strip of card pointed at each end, so 

 that these two points are situated about five inches before my eyes, one 

 before the right eye, the other before the left. The length of the strip of 

 card must be equal to the distance between the centres of the eyes, which 

 is 68 millimetres for my own eyes. Looking now with the right eye in 

 the direction of the right point of that strip, and with the left eye in the 

 direction of the left point, I am sure to bring the eyes always into the same 

 position relatively to my head, so long as the position of the strip of card 

 on the wooden piece remains unaltered. 



As a field of vision I use either a wall covered with a grey paper, in the 

 pattern of which horizontal and vertical lines can be easily perceived, or a 

 drawing-board covered with grey drawing-paper, on which a system of 

 horizontal and vertical lines is drawn, as in fig. 1, and coloured stripes are 

 fastened along the line ab. 



Now the observer at first must endeavour to find out that position of 

 his eyes which we call the primary position. In order to do this, the ob- 

 server takes the wooden piece between his teeth, and brings his head into 

 such a position that his right eye looks to the centre of the coloured stripes, 

 in a direction perpendicular to the plane of the drawing. Then he brings 

 his head into such an attitude that the right end of the card-strip appears 

 in the same direction as the centre of the coloured stripe. After having 

 steadily looked for some time to the middle of the coloured stripe, he turns 

 away his gaze to the end of either the vertical or horizontal lines, ab, cd, 

 which are drawn through the centre of the coloured stripe. There he will 

 see an ocular spectrum of the coloured stripe, and will observe if it coin- 

 cides with the horizontal lines of the drawing. If not, he must alter the 

 position of the strip of card on the wooden bar to which it is fastened, till 

 he finds that the ocular spectrum of the coloured stripe remains horizontal 

 when any point either of the line ab or cd is looked at. When he has 

 thus found the primary direction of his visual line for the right eye, he 

 does the same for the left. 



The ocular spectra soon vanish, but they are easily renewed by looking 

 again to the centre of the stripes. Care must be taken that the observer 

 looks always in a direction perpendicular to the plane of the drawing 

 whenever he looks to the centre of the coloured stripe, and that he does 

 not move his head. If he should have moved it, he would find it out im- 

 mediately when he looks back to the strip, because the point of the card- 

 strip would no longer cover the centre of the coloured stripe. 



So you see that the primary direction of the visual line is completely 



