357 



read a paper on Binocular Vision at the Royal Society on Monday. 

 As he has published his experiments and views on this subject in 

 three articles in Silliman's Journal for Juli/, October, and November, 

 1856, I presume that the paper he is about to read will contain the 

 same views. I regret that I cannot be at the meeting on Monday 

 to defend my theory of the Stereoscope against his objections to it ; 

 which are founded on an inaccurate perception of the phenomena, 

 and stand in direct opposition to the Law of Visible Direction, 

 which I have placed beyond a doubt, and which, I believe, is univer- 

 sally admitted. 



" Mr Rogers maintains that two lines of unequal length, AB, ab, 

 for example, ab being the shortest, can be made to coalesce perfectly, 

 i.e., that when the points A a are united by distinct vision, B and b 

 are also united. Now, when the optical axes are converged, on Aa 

 united and seen distinctly, B and b, the other ends of the lines, are 

 seen indistinctly, and, therefore, the observer cannot see them united, 

 unless by running the point of distinct vision from A to B, when he 

 will see them united. But when he is thus seeing these points B 

 and b united, A and a have separated till the eye returns and unites 

 them as before. This is the true process which goes on, and the ap- 

 parent union of the lines thus effected is aided by two causes which 

 Mr llogors does not seem to have noticed. The eye runs from A to 

 B and back again in less than one-third of a second (the duration of 

 the impression of light upon the retina), so that the impression of 

 A and a united remains when the eye is actually seeing B and b 

 united. The other cause is merely an auxiliary one, and is not neces- 

 sary to the apparent union of the line. It is the mental recollection of 

 the union of A and a when the eye has passed in an instant to join 

 B 6. I lay no stress, however, upon this fact, as it is only a phy- 

 sical one, on the supposition that a recollected impression is the re- 

 sult of a visual sensation. 



•' If two unequal lines can be united and perfectly coalesce, then 

 two separate visible points would have their pictures on the retina 

 coincident ; or, what is the same thing, a li7ie joining two points, a 

 and 6, would have a single point for its image on the retina ; and, 

 what is still more absurd, two different points of the retina would 

 have the same lino of visible direction ! 



" When the difference between the two lines AB and ab exceeds 

 a certain quantity, the apparent coalescence, produced by the causes 



VOL. III. 2 G 



