180 Prof. J. LeConte on some Phenomena 



in the two eyes. The horizontal line of demarcation is that me- 

 ridian of the eye upon which, under similar circumstances, the 

 image of an apparently horizontal line falls. The plane which 

 passes through the two visual lines we will call the visual plane, 

 and that visual plane which is at right angles to the line of the 

 face the primary visual plane. The line joining the root of the 

 nose and the point of sight, and which therefore bisects the angle 

 of optic convergence, we will call the median line of sight. 



Now Helmholtz gives as the law controlling all the move- 

 ments of the eye the following, viz. that when the eye turns 

 from its primary to any secondary position, it turns " on a fixed 

 axis which is normal both to the primary and to the secondary 

 visual line." In other words, the eye may turn on any axis at 

 right angles to the optic axis, but does not rotate about the optic 

 axis. Again, he states that " vertical and horizontal lines keep 

 their vertical or horizontal position in the field of vision when 

 the eye is moved from its primary direction vertically or hori- 

 zontally." This law had been previously stated by Listing, but 

 without proof; Helmholtz claims to have established it by ex- 

 periment. His method is very ingenious. It is well known 

 that if we look for some time at a bright object, and then turn 

 the eye upon a comparatively obscure field, a spectrum having 

 the form of the object will be seen. As such spectra are the 

 result of a temporary modification of the retina itself, they must 

 follow the motions of the eye with the greatest exactness. If 

 therefore the bright object be a line, then if there be any rota- 

 tion of the eye on the optic axis, in turning the eye in various 

 directions the linear spectrum ought to incline to one side or the 

 other. Suppose, then, the object be a bright-red vertical line 

 on a grey wall at the exact height of the eye : Helmholtz finds 

 that on gazing at the bright line with one eye, taking care that 

 the eye shall have its primary direction, and then turning the 

 eye in a horizontal plane to the right or left, the spectrum retains 

 perfectly its verticality. " I found/' he says, " the results of 

 these experiments in complete agreement with the law of List- 

 ing." For the ingenious device of Helmholtz for getting the 

 primary position of the eye we must refer the reader to his lec- 

 ture. I have tried Helmholtz's experiments with similar results. 

 Nevertheless I believe it may be demonstrated that though rota- 

 tion of the eye does not take place under the circumstances of 

 these experimeuts, yet it does so under other circumstances not 

 touched by them, and that in a manner which deeply affects the 

 question of the horopter. The law of Listing is doubtless true, 

 or nearly true, when the eyes move together parallel to each 

 other, but is far from being true in strong convergence. The 

 experiments which follow prove beyond a doubt that in my own 



