72 On Single and Double Vision, and an Optical Illusion. 



Having measured the distance between my eyes, 2*6 inches, 

 the distance between the figures on the wall 21 inches, the dis- 

 tance from the wall ten feet, the distance of the illusive image 

 was calculated to be 14*7 inches, when it had been measured as 

 near as may be 14*5 inches. In a second experiment we endeav- 

 ored to ascertain the distance of the observer from the wall. 

 The other data were : 



Distance between the eyes, . . . 2*6 inches. 



Distance between the figures, ... 21 inches. 



Distance of illusive image, . . . 1675 inches. 



Calculated distance of the wall, . . . 12*5 feet. 



Measured distance of wall, . . , 13 15 feet. 



When it is recollected that the observer is obliged to range 

 lengthwise on his measure while he determines the distance of 

 the aerial image, and that the base line is only 2*6 inches, the 

 above results appear quite as accurate as we ought to anticipate. 



There is peculiar beauty and accuracy in some of the results 

 of these experiments, and it had occurred both to Sir David 

 Brewster and myself, that when a strip of wall paper was placed 

 at a greater or less distance from its fellow than others, the illu- 

 sive image would not appear in the same place, some strips would 

 advance a little and others would recede, so as to fulfill the con- 

 ditions of the triangles above named* even the sixteenth of an 

 inch would be appreciable. 



In the history of my examination of this subject, I would ob- 

 serve that my friend Dr. D. S. C. H. Smith, of Sutton, was pres- 

 ent when my paper was read at New Haven. In 1845, my as- 

 sistant, Thos. K. Beecher, A.M., witnessed and repeated most 

 of the experiments above named. Among other things we 

 made the equations dependent upon the above triangles, and veri- 

 fied our calculations by actual admeasurement of the distances 

 between the eyes, between the objects, and to the illusive image. 

 I attempted a popular lecture on this topic, but found it difficult 

 to interest an audience in a matter requiring so much previous 

 optical knowledge. In the spring of 1846, I communicated the 

 leading principles of what I thought then questionable discove- 

 ries, either to Prof. Bache or to Prof. Henry, and consulted 

 him as to their originality. He gave his opinion that they were 

 new. Without the least disposition to contest the point of orig- 

 inality, which I have failed to establish by neglecting to publish 

 my results, I wish merely to inform my friends of what I have 

 m fact done, and thus appear as a collateral witness to the truth 

 and interest of Sir David Brewster's paper. He has brought 

 forward some points which had never presented themselves to 

 me. That figures less distant than the two eyes may be so view- 

 ed as to form an illusive image at a greater distance than the 

 object itself, is evidently true, yet I had never made or anticipa- 



