ii« Sir C. Blagden's Appendix to 



opportunity of relating an experiment made many years ago, 

 to decide how far the similarity of the images seen by each eye 

 contributed to make them impress the mind as one. In the 

 house where I then lived was a marble chimney-piece, the 

 upper horizontal block of which was fluted vertically; and 

 the ridge between each concavity of the fluting was about as 

 wide as the concavity itself. When I looked at this range of 

 fluting at the distance of about nine inches, and directed the 

 optic axes to it, I saw of course every ridge and concavity 

 distinctly, and judged rightly of the distance. Adjusting the 

 optic axes as to an object a little further o% I discerned the 

 fluting confusedly and all double, the ridges interfering with 

 the concavities ; which was accompanied with the uneasy sen- 

 sation of squinting. But on widening the direction of the optic 

 axes still more, as to an object about eighteen inches distant ; 

 (namely, just so far that the duplication of the images should 

 correspond successively ; that is, so that the first ridge and 

 concavity of the fluting, as seen by one eye, should fall in with 

 the second ridge and concavity, as seen by the other;) the 

 fluting appeared as distinct and as single as at first ; but it 

 seemed to be about double the distance from the eye that it 

 really was, and to be magnified in proportion ; nor had I, in 

 this case, any sensation of squinting. As the parts of the 

 fluting, though in general much alike, were not exactly so 

 every where in colour and minute circumstances, there ap- 

 peared in some places a slight confusion from this dissimilarity 

 of the images ; but that trifling confusion had no manner of 

 eflf'ect on the mind's judgment of the images, which looked 

 as perfectly single, as when the fluting was viewed with the 

 optic ax^s so directed, that the ridges and concavities ^^oxi by 



