88 '/. LeConte an some phenomena of Binocular Vision. 



I will DOW give a few very simple experiments and the visual 

 results, represented in tlie manner already indicated. Let a 

 smooth white plane about twenty inches long and of any con- 

 venient width, be prepared bv pasting white paper on a smooth 

 flat board, (fig. 12). Mark two points E and S with a space 

 between them exactly equal to the interocular space. In the 

 middle between the two points notch the end of the board so 

 as to fit over the bridge of the nose. From the notch n draw 

 the line nA in the position of the median line of sight when 

 the notch is fitted on the nose, and on this line place pins at A 

 and B. If now, the plane be fitted on the nose and held in a 

 horizontal position a little hehw the visual plane, so that both 

 the pins and the line can be distinctly seen, and the eyes be di- 

 rected upon a distant object ; then, as already stated the median 

 line will be seen as two parallel lines separated by a space equal 

 to the interocular space. If in addition to the median line the 

 position of the visual lines be represented by dotted lines, then 

 fig. 13 will represent the actual relation of parts, and fig. U 

 the visual result. It will be observed by comparing the visual 

 result with the drawing, that the parts of the face, the whole 

 plane and all the objects and lines on the plane are shifted by 

 the right eve one half the interocular distance to the left, and by 

 the left eye the same distance to the riglit ; the two visual lines 

 of the drawing are combined in the result to form a true median 

 binocular visual line EV, while the median line of the drawing 

 is doubled, forming heteronymous images, which in the result 

 occupy the position of the visual lines of the drawing. Besides 

 these three lines, there are seen two other dotted lines Iv and r'v'. 

 These are not visual lines proper, i. e. lines along which vision 

 takes place, but they are the visible representatives of the visual 

 lines of each eye seen as objecis by the other eye. 



Next, having removed the dotted lines in the previous experi- 

 ment, let the eyes be directed upon the pin A, and dotted lines 

 be again drawn in the ^direction of the visual lines as in fig. 15- 

 The visual result is shown in fig. 16. By comparing this visual 

 result with the actual drawing it will be seen that the whole 

 triangle L A R, is turned upon A as a fixed point a half inter- 

 ocular distance to the left bv the right eve, and to the right by 

 the left eye. 



Next, having again erased the dotted lines of the previous 

 experiment, and substituted df)tted lines crossing at B as in 

 fig 17; let the plane be auain adjusted to the nose and the eves 

 du-ected upon the pin B. The visual result is shown in fig. 18- 

 In this visual result the whole drawino- ((i<v. 17) is apparently 

 rotated about the ]H)int of sight B in opposite directions by 

 the two eyes, through a distance represented bv the ande ?iBB 

 or nBL -half the visual angle. As (^bj.M-ts (^ii the nearer and 

 the farther side of the point^B must move in opposite directions, 



