THE SENSE OF VISION. 



357 



pair across the end of a piece of brass tube about one inch or more in diameter 

 and ten inches long:. Let the tubes be held one in front of each eve with the 

 distant ends nearly in contact with each other, as shown in Figure 174. If 

 the tubes are in such a position that the small circles are brought as near to 

 each other as possible, as shown in Figure 173, the retinal images will blend, 



the smaller circle will seem to be much 

 nearer than the larger one, and the eyes will 

 appear to be looking down upon a truncated 

 cone, such as is shown in Figure 175, since 

 a solid body of this form is the only one 



Fig. 174. — To illustrate stereoscopic vision. 



Fig. 175.— To illustrate stereoscopic vision. 



bounded by circles related to each other as those shown in this experiment. 



Stereoscopic slides often serve well to illustrate the superiority of binocular 

 over monocular vision. If the slide represents an irregular mass of rocks or 

 ice, it is often very difficult by looking at either of the pictures by itself to 

 determine the relative distance of the various objects represented, but if the 

 slide is placed in the stereoscope the true relation of the different parts of the 

 picture becomes at once apparent. 



Since the comparison of two slightly dissimilar images received on the two 

 retinas is the essential condition of stereoscopic vision, it is evident that if the 

 two pictures are identical no sensation of relief can be produced. Thus, when 

 two pages printed from the same type or two engravings printed from the same 

 plate are united in a stereoscope, the combined picture appears as flat as cither 

 of its components. If, however, one of the pictures is copied from the other, 

 even if the copy be carefully executed, there will be slight differences in the 

 distances between the lines or in the spacing of the letters which will cause 

 apparent irregularities of level in the different portions of the combined pic- 

 ture. Thus, a suspected banknote may be proved to be a counterfeit if, when 

 placed in a stereoscope by the side of a genuine note, the resulting combined 

 picture shows certain letters lying apparently on different planes from the rest. 



Peeudoscopic Vision. — If the pictures of an ordinary stereoscopic slide be 

 reversed, so that the picture belonging in front of the right eve is presented to 

 the left eye, and viceversd, the stereoscopic gives place to what is called a pseudo- 

 scopic effect — i. e. we perceive not a solid but a hollow body. The effect is best 



