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 July, October, and November^ 

 1855, 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 ray 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 Rogers 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 line 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 line 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 



