SIR DAVID BKEWSTER ON BINOCULAR VISION. (575 



APPENDIX. 



When I wrote the paragraph in page 647, 1 had no expectation of learning that 

 any example of such an illusion had ever occurred. A friend, however, to whom 

 I had occasion to shew the experiments, and who is short-sighted, mentioned to 

 me that he had 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-bottomed 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 me 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 stand- 

 ing out upon me, and waving, as you describe, 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 find- 

 ing 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." 



VOL. XV. PART IV. 8 U 



