ESTIMATION OF SIZE AND OF DISTANCE. 



the observer; in the other case (I) the opening of the angle is toward the observer. 

 In order that both eyes f and i (in I) may see the object a, the eyes must converge 

 more than if they were turned directly toward a, because the glass plates displace 

 the rays a c and a g parallel with each other (e f and h i) . Therefore the object 

 appears nearer and smaller at a. In II the rays b : k and bj o from the smaller, 

 nearer object b t fall on the glass plates. In order to see the object b lf the eyes 

 (n and q) must diverge more, and the object appears at f , enlarged and more distant. 

 3. By examination of Wheatstone's stereoscope (Fig. 305, II), it will readily be 

 seen that the nearer the two pictures are brought to the observer, the more the 

 latter must converge (because the angles of incidence and reflection become 

 larger). Therefore, the fused image seems smaller. If the middle of the picture 

 R is moved to R 1; the angle S u r f must be made equal to Sj r R : (likewise naturally 

 on the left). 4. As, in using the telestereoscope, the eyes are, in a manner, moved 

 far apart, they must be converged more strongly, in looking at objects at a certain 

 distance, than in normal vision. Objects in the landscape, therefore, appear as 

 in a small model. As, however, it is customary to consider the distance great 

 when the objects are so small, the latter appear to be moved at the same time 

 to a remarkable distance. 



With respect to the judgment of distance the following rules may be 

 noted: With retinal images of equal size, the distance is estimated to be 



the greater the less the effort of ac- 

 commodation (and conversely). In 

 binocular vision, with retinal images of 

 equal size that object is judged to be 

 furthest away that requires the least 

 convergence of the visual axes (and 

 conversely). 



The estimation of size and distance, 

 therefore, go largely hand in hand, and 

 the correct judgment of distance affords 

 also a correct estimate of the size of 

 objects. A further aid to the estima- 

 tion of distance is furnished by the ob- 

 servation of the apparent displacement 

 of objects on movement of the head or 

 body. During such movements , ob j ects 

 are apparently displaced laterally more 

 rapidly the nearer they are. For this 

 reason, in traveling in an express 

 train, the objects change their position 



h great rapidity, and they appear nearer and consequently smaller 

 than they are. 



Finally, those objects appear nearest that are most distinct in the 

 neld. or vision. 



9 w ' (b 



T ' " 



I I 



FIG. 308. Rollett's Glass Plate Apparatus. 



f A Hght in a dark landsca Pe, likewise a dazzling mountain-top 

 th ai Sn W ' ^^ exceedin Sly "ear. Viewed from a high mountain 



level of ^^ g 1^? mg ' Wm ^ m 1 S ^ reams not rarel > T a PP ear to be lifted a bove the 

 the landscape. On looking at the railway embankment from a train, 



fixed g for ^ P t aSS f mdlstmctl y b ^ore the eyes. If suddenlv a certain point is 

 levd towafey V e 1S1 n ' * ^^ ""^ to project from the general 



tefmlfdiafp^f 8 f Size 1 and Direction. i. A given distance filled out by 

 Sre X ,W ? a PP ears lar ? e r than the same distance without them. There- 



the dit of^F 6 ^ 8 elll P tlca1 ' mstead of hemispherical; and for the same reason 

 2 If a oirHP^ 1 ^ na PP ea ^s larger than when it is high in the heavens. 



eilir.se I < ft f^n ed / ow fe to and fr behind a slit ' * a PP ears as a horizontal 

 be drawn ohTn T rapidly, it appears as a vertical ellipse. 3. If a fine line 

 be drawn obliquely across a heavy, black, vertical line, the former appears to 



