492 PROCEEDINGS OF THE SOCIETY. 



nearer (its retinal picture remaining of the same size) — it seemed to 

 become smaller ; while, by opening out the angle of convergence, 

 the pictures seemed to grow larger. Here, then, we have a most 

 perfect example of an automatic mental interpretation, in which the 

 apparent size is determined by the conjoint impressions we are 

 receiving from the convergence of the optic axes and the actual 

 size of the retinal pictures. And so when the mental interpretation 

 of the stereoscopic form throws the small square back, and the visual 

 picture remains of the same size, the fact of its receding without 

 diminishing suggests the mental impression that it is of a really larger 

 size. There is no gainsaying these things ; they are simply facts in 

 Mental Physiology ; and, as I have already said, they are not a matter 

 of Optics, but the results of a mental process of interpretation of the 

 visual impressions received. 



I might go on to demonstrate this still further by means of the 

 Pseudoscope, if time permitted. This is an arrangement of prisms 

 for bringing the right-hand picture of an actual object to the left eye, 

 and the left-hand picture to the right eye ; and just as the " crossing " 

 of two stereoscopic pictures produces a conversion of relief in the com- 

 posite image, so does this reversal of the combination suggest to the 

 mind a reversal of the relief of the image of the actual object. The 

 effect of " suggestion " is well shown by a simple experiment. Here 

 is an ordinary tin cake-mould ; now if you place this before one eye 

 so that the light falls directly into it, and you look at it with that eye 

 alone, inasmuch as you are more accustomed to see the solid form than 

 the hollow, you will probably see it projecting towards you. (To see it 

 in this way, there must be no shadow, for this will oblige you to 

 recognize its concavity.) The experiment is best made by daylight, 

 the mould being held up so as to face the person looking into it with 

 his back to a window. The picture that falls on his retina is virtually 

 that of a flat surface ; seeing it as such, he has to interpret the meaning 

 of that picture ; and as he is more accustomed to see the solid form than 

 the hollow mould, the latter is preferentially suggested to his mind. 

 As another very curious instance of this kind of suggestion, I have 

 here a mask, which I long ago got one of my sons to paint inside, 

 just in the same way that the outside is usually painted. If this is 

 held up so that there is no shadow, and a person looks into it steadily 

 with one eye, the mental impression is that of projection. I was 

 about to write a paper on Binocular Vision and the Stereoscope 

 ior the ' Edinburgh Keview,' and I asked the editor to come to my 

 residence and see a few experiments. I placed him with his back to 

 the window, and then, holding up this mask with the inside towards 

 him, so that the light fell into it without causing shadow, I asked 

 him to look at it with one eye, and to say what he saw. He said at once 

 that he saw the face of an ordinary mask. I then told him to open 

 the other eye, and he was utterly astonished to find that he had been 

 looking at the inside. Sir Charles Wheatstone told me that by long 

 looking at a bust with his Pseudoscope, he had been able to reverse 

 its relief; but that he could never do so with a living human face. 

 And I have found that although, by crossing the pictures in the 



