42 THE POPULAR SCIENCE MONTHLY. 



ing but a single eye, that looks into the very depths of the observer's 

 Cyclopean eye. 



The conception of this subjective union as the product of the ex- 

 perience of the race in interpreting sensations, and the consequent 

 necessity of distinguishing between realities and their visual represen- 

 tations, seems never to have been appreciated until long after the in- 

 vention of instruments for use in the analysis of vision. Much con- 

 fusion has resulted from the attempt to explain what are really sub- 

 jective results of retinal sensation by the application of geometric 

 principles, irrespective of the illusive union of the two eyes when em- 

 ployed together. In 1604 Kepler stated that the 'distance between the 

 eyes constituted a base-line, which we employ for measuring the dis- 

 tance of objects by a species of visual triangulation. This idea was 

 subsequently greatly elaborated by Sir David Brewster and others ;. 

 and in most, if not all, of our text-books of physics to-day it is applied 

 in a very familiar diagram to explain the principle of the stereoscope. 

 On this theory the apparent position of every point in the stereoscopic 

 field of view is determined by the meeting of separate visual lines, 

 which converge in front. An obvious consequence is that this locali- 

 zation should become impossible if the visual lines become parallel or 

 divergent. But, in truth, there can be no perception of locality by 

 this method. If the eyes are subjectively united, the visual lines be- 

 come subjectively united along with them ; if, indeed, such language 

 is at all applicable to lines that are mere abstractions. In its applica- 

 tion to stereoscopic vision, therefore, the diagram is worthless ; for 

 such vision is much easier to most persons when the visual lines are 

 parallel, or very slightly divergent, than when they are strongly con- 

 vergent, and in no case can there be any recognition of intersection 

 between lines which, if subjectively perceived at all, w T ould be coinci- 

 dent throughout their whole extent. 



The error just mentioned has undoubtedly sprung from the assump- 

 tion that stereoscopic vision is always perfectly normal. If this be so, 

 it should be as painless as the reading of this page, even when con- 

 tinued for hours in succession. Every one who has tried the experi- 

 ment with an ordinary stereoscope, and a large, miscellaneous collec- 

 tion of stereographs, knows how wearying it is, and how in some cases 

 distinct vision is found impossible. To indicate the real differences 

 between normal vision and that which is attained in most stereoscopes, 

 it will be necessary first to study the development of this instrument. 



The duality of human vision of near objects, and the consequent 

 dissimilarity of retinal pictures in the separate eyes, was apprehend- 

 ed and more or less vaguely discussed by Euclid (b. c. 300), Galen 

 (a. d. 200), Baptista Porta (1593), Leonardo da Vinci (1584), Aguilo- 

 nius (1613), and by Smith, Harris, and Porterfield during the eight- 

 eenth century. No practical results were wrought, however, until 

 1838, when Sir Charles Wheatstone read before the Royal Society his 



