﻿On some Phenomena of Binocular Vision. 131 



both substances are solid, metallic, and of a white aspect ; that 

 the alloy contains aboat 20 volumes of palladium united with 1 

 volume of hydrogenium, and that the density of the latter is 

 about 2, a little higher than magnesium, to which hydrogenium 

 may be supposed to bear some analogy ; that hydrogenium 

 has a certain amount of tenacity, and possesses the electrical 

 conductivity of a metal ; and, finally, that hydrogenium takes 

 its place among magnetic metals. The latter fact may have its 

 bearing upon the appearance of hydrogenium in meteoric iron, 

 in association with certain other magnetic elements. 



I cannot close this paper without taking the opportunity to 

 return my best thanks to Mr. W. C. Roberts for his valuable co- 

 operation throughout the investigation. 



XVIII. On some Phenomena of Binocular Vision. By Joseph 

 LeConte, Professor of Chemistry and Geology in the Uni- 

 versity of South Carolina *. 



I. Adjustments of the Eye. 



TWO kinds of ocular adjustment take place in every volun- 

 tary act of sight, viz. (1) a proper convergence of the optic 

 axes so that they shall meet on the object of sight, and (2) an 

 adjustment of each eye so that the diverging pencil of rays which 

 enters the pupil shall be brought to perfect focus, and therefore 

 produce a perfect image on the retina. The first or binocular 

 adjustment is necessary for single vision ; the second or focal ad- 

 justment is necessary for distinct vision. The first is distinctly 

 sensible for all distances within 100 yards, and perhaps for much 

 greater distances ; the second is scarcely, if at all, sensible for 

 distances beyond two yards. 



To the two adjustments mentioned above may be added a 

 third, viz. contraction of the pupil. The design of the contrac- 

 tion of the pupil is probably to increase the clearness of defini- 

 tion of the retinal image by cutting off the most divergent rays 

 from very near objects, and thus to decrease the spherical aber- 

 ration which is not entirely corrected in the eye by the form of 

 the lens. The pupil, however, also contracts involuntarily under 

 the stimulus of strong light, without regard to distance. This 

 must be carefully distinguished from the adjustive contraction, 

 which is (to some extent at least) voluntary. 



These three adjustments of the eye, viz. binocular or axial 

 adjustment, focal adjustment, and contraction of the pupil, are 

 associated in every voluntary act of sight. They are accom- 

 plished by one act of volition. They are so intimately associ- 

 ated that they cannot be voluntarily separated. It is usually 

 * From Silliman's American Journal for January 1869. 

 K2 



