318 On the Knowledge of Distance given hy Bi?iocidar Vision. 



and the lines M N, O P will converge to a vanishing point 

 between M O and the observer. 



These results may be considered as laying the foundation 

 of a new art, to which we may give the name of Visual Per- 

 spective, in contradistinction to Geometrical Perspective. 

 This art furnishes us with an immediate explanation of a gi'eat 

 variety of optical illusionswhich have never yet been explained; 

 and there is reason to believe that some of its principles were 

 known to ancient architects, and even employed in modifying 

 the nature and position of the lines and forms which enter into 

 the construction of their finest edifices. 



St. Leonard's College, St. Andrews, 

 April 10, 1844. 



Appendix. 



When I wrote the paragraph in page 314, I had no expec- 

 tation of learning that any example of such an ilUision had 

 ever occurred. A friend, however, to whom I had occasion 

 to show the experiments, and who is short-sighted, mentioned 

 to me that he liad been on two occasions greatly perplexed 

 by the vision of these suspended images. Having taken too 

 much wine, and being in a papered room, he saw the wall 

 suspended near him in the air; and on another occasion, 

 when kneeling and resting his arms on a cane-bottoined chair, 

 he had fixed his eyes on the carpet, which accidentally united 

 the two images of the open-work, and threw the suspended 

 image of the chair-bottom to a distance, and beyond the plane 

 on which his arms rested. 



The following case, communicated to nie by Professor 

 Christison, is still more interesting. " Some years ago, when 

 I resided in a house where several rooms are papered with 

 rather formally recurring patterns, and one, in particular, with 

 stars only, I used occasionally to be much plagued with the 

 wall suddenly standing out upon me, and waving, as you de- 

 scribe, with the movements of the head. I was sensible that 

 the cause was an error as to the point of union of the visual 

 axes of the two eyes ; but I remember it sometimes cost me a 

 considerable effort to rectify the error ; and I found that the 

 best way was to increase still more the deviation in the first 

 instance. As this accident occurred most frequently while I 

 was recovering from a severe attack of fever, I thought my 

 near-sighted eyes were threatened with some new mischief; 

 and this opinion was justified in finding that, after removal 

 to my present house — where, however, the papers have no 

 very formal pattern — no such occurrence has ever taken place. 

 The reason is now easily understood from your researches." 



