246 W. LeConte 8te-vrtt.s I*lii/.sh>lo<ji<-uJ Optic*. 



apparent combination of one line with two other lines at the 

 same time was noticed by Professor W. B. Rogers in 1856 and 

 further discussed by Helmholtz in 1867, but its bearing on the 

 theory of binocular perspective has not received sufficient 

 attention. It not only shows the insufficiency of the theory 

 upheld by Briicke and Brewster, but also seems to indicate 

 that, if there be any intuitive power of distinguishing between 

 heteronymous and homonymous double images, this power 

 must be understood to extend to cases in which a ain 

 belongs to both kinds of double image at the same time. If 

 we admit intuition at all in this connection, we must further 

 grant that a distinction can thus be made instantly between 

 these opposite kinds of images, even when they are so minute 

 that the unaided eyes cannot separate them through .an act of 

 conscious judgment ; and that this is habitually done by thous- 

 ands who throughout life fail to suspect even the existence of 

 such duplication in any part of the field of view. If there is 

 any perception of double images, even when the interval be- 

 tween the components is wide, it is by an act of special atten- 

 tion. Many of our judgments, not only in vision but in the 

 performance of other bodily functions, 'are instantaneous and 

 unconscious; but probably we shall never be able to put an 

 exact dividing line between those due to the experience of the 

 individual and those that spring from tendencies transmitted 

 by the race. We learn to see, just as we learn to walk or talk 

 in infancy, by oft-repeated efforts which form a succession of 

 experiences. If passive seeing be a result of mere inheritance, 

 then active looking is superadded as a result of training. The 

 empirical theory, if sufficient to explain all known facts of vis- 

 ion, can leave no room for intuition, and its resources must be 

 exhausted before any resort to intuition can be deemed necessary. 

 To test the illusions of abnormal vision by the light of the 

 electric spark, a series of experiments has been made in which 

 the optic angle was varied from 3° of divergence to 50° of 

 convergence of visual lines. The graduated reflecting stereo- 

 scope was employed, the. mode of experiment being the same 

 as that described in a former paper, 1 ator of the 



apparatus keeping the observer's record of estimates. Mr. 

 Share and myself relieved each other by turns, taking care to 

 avoid fatigue of the muscles of the eyes, each remaining igno- 

 rant of his own record until the entire series of experiments 

 was completed. After the arms of the stereoscope had been 

 arranged to necessitate a particular value of the optic angle, 

 the observer opened his eyes, and, by the light of a slow suc- 

 cession of sparks, adjusted them to secure binocular vision of 

 the stereograph of the moon, each picture of which was kept 

 in a fixed position on the arm that carried it. The observer's 



