Vision by the Lhjht of th< K/ectrlc Spark. 24 



scious or unconscious. They are exceedingly convenient f< 

 the purpose of explaining binocular vision, but so limited a 

 explanation can never cover all the facts. 



In 1864 Professor C. F. Himes described a pair of pictur< 

 which he had devised to illustrate the variation in apparer 

 size of the moon when viewed successively at the horizon an 

 the zenith. Within two equal circles, A and A', fig. 2, ai 



. on opposite 

 . combining 



circles, B, B' and B", the latter equal to each 

 I are concentric, while the centers of B' and B" 



.ides of that of A 7 and aligned with that of A. 



he two pictures binocularly, A and A' at once 

 when united form a small circle whose plane 

 is nearer, B and B" one whose plane is farther than that of A 

 and A', assuming the union to be by diminished convergence 

 "f visual lines. The circle BB" appears as much larger than 

 BB' as its dislai, ee is judged to be greater, despite the fact that 

 the small circles are all of equal size. On the theory of double 

 images, when I'd}' is regarded, the circle IV remains uncom- 

 bined while AA' should be seen double; the comparison of 

 the nearer and more remote combinations being attained by 

 -■ them successively. But in fact the appearance of 

 the three circles, each at its proper distance, is instantaneous 

 and simulta mve. If the observers eyes are 



well trained the circle AA' may be detected as double while 

 the others are seen single. If the gaze be very rigidly fixed 

 upon the center of the combined circle AA', the others are 

 separated, three small circles are seen, but all apparent varia- 

 tion in distance is lost. This stereograph has been examined 

 under the light, of the electric spark by both Mr. Share and 

 results being entirely analogous to those obtained 

 with continuous light. Kacii circle was appropriately marked 

 and the cards adjusted by the manipulator in such maimer that 

 the observer could have no previous knowledge to aid him in 

 ig which combined circle ought to appear nearest. 

 A single spark was usually sufficient to show each in its appro- 

 priate place. The circle \) has the same effect on B' and B" as 

 if it were simultaneously combined with both of them. This 



