1 86 UNIVERSITY OF COLORADO STUDIES 



much fascinated by the scene as if he were standing on the spot with 

 Roosevelt and actually beholding the view. 



The third picture represents a photograph taken up a tree. This 

 picture, as a photograph, looks like a wigwam, and by many is not recog- 

 nized at all; but just as soon as the stereograph is held up and the head 

 thrown back as if actually looking up the tree, when the two pictures 

 of the stereograph are superimposed, the effect is exactly the same as 

 if actually standing at the foot of the tree and tooking up into its top. 

 In merely looking at this view, without looking up, you will not get the 

 same interpretation. Throwing back the head is absolutely necessary. 

 Unless you do, all the factors are not present that would be present 

 were you actually beholding the scene. 1 



I hope I have made clear the fact of the valuable aid which the 

 stereoscope lends in working out the factors of vision. Before I close 

 I shall outline just a few more experiments to show that in every case 

 we perceive in terms of normal vision. Fig. 16 shows that in normal 

 vision the rays of light entering the eye cross in the pupil. The pupil X 

 is a pinhole in the iris cc. When there is strong light the iris contracts 

 and narrows this opening, whereas when it is weak it relaxes and enlarges 

 the pupil. The figure shows that as a consequence of the crossed rays 

 the image is always (in normal vision) inverted. In Fig. 18 we form 

 an artificial pupil at X. This is done by looking at a pin through a 

 pinhole in a piece of cardboard held up close before the eye. The pin 

 is not inverted but will appear to be in the hole as in M, accompanying 

 this figure. When now the pin is held between the eye and the pin- 

 hole, Fig. 21, the image of the pin-head will not be inverted, consequently 

 the eye will interpret as in normal vision and see it as upside down in 

 the pinhole, as in M of this figure. In both these last cases there is an 

 illusion due to the fact that interpretation in terms of the factors of 

 normal vision in these selected cases is not valid. 



Stratton tried an experiment of tying lenses over the eyes in such a 

 way as to bring the image of the object looked at upright instead of 

 inverted, and for a number of days everything was topsy-turvy. But 

 gradually he became adjusted, and had he continued wearing the lenses, 



1 These last three stereographs are published by permission of Underwood and Underwood, New York. 



