Page:Mind (Old Series) Volume 12.djvu/355

 342 w. JAMES : cut off by screens so as to prevent all suggestions of perspec- tive, &c. The three objects in each experiment seem in the same place. 1 Add to this the impossibility, recognised by all observers, of ever seeing double with the fovece, and the fact that authorities as able as those quoted in the note on Wheat- stone's observation, deny that they see double then with identical points, and we are forced to conclude that the pro- jection-theory, like its predecessor, breaks down. Neither formulates exactly or exhaustively a law for all our percep- tions. What does each theory try to do ? To make of seen loca- tion a fixed function of retinal impression. Other facts may be brought forward to show how far from fixed are the perceptive functions of retinal impressions. We alluded a while ago to the extraordinary ambiguity of the retinal image as a revealer of magnitude. Produce an after-image of the sun and look at your finger-tip ; it will be smaller than your nail. Project it on the table, and it will be as big as a strawberry ; on the wall, as large as a plate ; on yonder mountain, bigger than a house. And yet it is an unchanged retinal impression. Prepare a sheet with the following figures strongly marked upon it, and get by direct fixation a distinct after-image of each. Fig. 4. Project the after-image of the cross upon the upper left- hand part of the wall, it will appear as in Fig. 5; on the upper right hand it will appear as in Fig. 6. The circle similarly Fig. 5. Fig. 6. Archivf. Ophthal, Bel xvii., Abth. 2, pp. 44-6 (1871).