192 Prof. J. LeConte on some Phenomena 



line of demarcation. Both the rotation and the change in the 

 relation of the two lines of demarcation increases with the degree 

 of optic convergence. It is possible that the frequent action of 

 the muscles distorting the globe of the eye may leave some per- 

 manent impress upon the form of the globe, so that even in a 

 passive state the vertical line of demarcation does not coincide 

 perfectly with the vertical meridian. If so, then to that extent 

 Helmholtz's principle of real and apparent vertical in the primary 

 position of the eye will be true. Or, to express it differently, 

 we have seen that the inclination of the vertical upon the hori- 

 zontal line of demarcation decreases as the point of sight recedes ; 

 at 1| inch it is 5°, at 2*2 inches it is 1^°, at 7*5 inches it is 25', 

 and at 12 inches 20'. It is possible that even when the point of 

 sight recedes to infinite distance, and the horizontal line of de- 

 marcation becomes coincident with the horizontal meridian, the 

 vertical line of demarcation may still make a small angle with 

 the vertical meridian. If so, this angle is the difference between 

 the real and apparent vertical spoken of by Professor Helmholtz. 

 We do not yet admit this as probable, however ; for the natural 

 position in which all lines at all distances combine when the 

 visual plane is inclined 45° downward seems inconsistent with 

 this idea. 



The decrease of the rotation of the eye when the visual plane 

 is inclined downward, and its increase when the visual plane is 

 inclined upward, seem to be the result of the relative power of 

 the two oblique muscles. Ordinarily the inferior oblique is the 

 stronger, and the rotation is therefore outward ; but as the visual 

 plane is inclined downward, the action of the two become more 

 and more nearly equal, until at 45° they balance each other and 

 there is no rotation. Below 45° the action of the superior 

 oblique predominates, and the eye therefore rotates inward. In 

 turning the visual plane upward and converging strongly, the 

 action of the inferior oblique predominates more and more. 



It will be observed that the rotation of the eye which we have 

 demonstrated necessitates, in optic convergence, a difference be- 

 tween the real and apparent vertical ; but our views differ entirely 

 from those of Professor Helmholtz in the following respects : — 

 (1) Professor Helmholtz admits only a difference between real 

 and apparent vertical ; we have shown a difference between the 

 real and apparent horizontal as well as the real and apparent 

 vertical. (2) Professor Helmholtz' s difference is a constant one, 

 viz. 1^°; ours varies from 11° to 20', and probably to zero. 

 (3) According to Professor Helmholtz, the relation of the appa- 

 rent vertical to the apparent horizontal is a constant one, viz. an 

 angle of about 88|°; our experiments prove that this relation 

 varies to the extent of 5°. 



