SINGLE VISION. 43 



axes, as its real situation has to the real situa- 

 tions of such lines. And the like will be found, 

 by observation, to be true of every other line, 

 which may be drawn through the point of inter- 

 section of the optic axes to the visual base. 



The whole of what has here been said may be 

 illustrated and confirmed, by having again re- 

 course to the experiments with strings of dif- 

 ferent colours. In formerly describing those 

 experiments, I did not mention all the appear- 

 ances which occurred upon making them, hut 

 only such, as had immediate reference to the 

 points then under consideration. When, for 

 instance, a red string was placed in the axis of 

 the right eye, and a green one in that of the 

 left, I said that they both appeared in the com- 

 rnon axis. But this is not the only phenomenon 

 to be observed with respect to their apparent 

 number in this experiment. For as the red 

 string is also seen by the left eye, and the green 

 by the right, two other strings become visible, 

 beside that in the common axis, the apparent 

 positions of both of which will be found to be 

 the same with those, which ought to follow from 

 the present proposition. iShould now a yellow 

 string be placed between the two former, as in 

 the proof of the second proposition, its appear- 

 ance to the right eye will bisect the space be- 

 tween the appearances of the red and green 



