440 Mr. W. Le Conte Stevens on Wheatstone and 



The optic vertex is simply Brewster's " binocular centre." 

 In normal binocular vision of near objects, the distance of the 

 point of sight is sufficiently nearly given by this formula, 

 while the subjective effect is that of the superposition of the 

 two retinal images in a central binocular (cyclopean) eye*, 

 the two visual axes being combined into a single median line, 

 along which distance is estimated. These lines in turn have 

 no objective existence; and their intersection really cannot 

 determine any external point. Any effect due to axial con- 

 vergence is merely the interpretation which experience has 

 taught the observer to put upon the sensation of muscular 

 tension, as the internal rectus muscles contract. This inter- 

 pretation may be strengthened or antagonized by what other 

 conditions suggest ; and the estimate of distance on the com- 

 bined median line is modified accordingly. The direction of 

 the combined image is along this median, whether the visual 

 axes be convergent, parallel, or divergent. 



If the conjugate pictures selected be the simplest possible, 

 so that all kinds of perspective are eliminated except that due 

 to stereoscopic displacement, there are still left three elements 

 to consider, which can never be entirely dissociated. These 

 are : — 



I. The optic angle, interpreted by the sensation of tension 

 in the rectus muscles of the eyeballs. 



II. The focal adjustment, interpreted by the tension in the 

 ciliary muscle encircling the crystalline lens. 



III. The visual angle, interpreted by recognition of the 

 retinal area impressed. 



In normal vision, these elements conduce to the same visual 

 effect without becoming separately manifested to the con- 

 sciousness of the individual. In the stereoscope they may 

 conflict among themselves ; and the result is modification of 

 judgment, attended with some uncertainty. 



Let us now assume the arms of our stereoscope to be so 

 turned upon their pivot as to change the value of a from 

 7° 20' to 37° 20', the interocular distance, a, being taken as 

 64 millim. Applying our formula, we find D = 10 centim. 

 The focal adjustment, however, must still be for an image 50 

 centim. distant, if distinct vision is attained. Strong contrac- 

 tion of the internal rectus muscles is habitually associated with 

 nearness of the point of sight, and accompanied with strong 

 ciliary contraction. But in the present case the effort to 

 secure distinct vision implies relaxation of the ciliary muscles, 

 the effect of which is to suggest greater distance of the point 



* Le Conte, ' Light/ New York, 1881, pp. 213-2G9; or American 

 Journal of Science, ser. 3, vol. i. p. 33 et seq. 



