BINOCULAR VISION. 



533 



a direction at right angles to the axis of the telescope, 

 so as to form the image a. 2 > 2 near the tube of the instru- 

 ment. At this place a smaller tube is attached, which 

 carries the eye-piece e g~ This lens forms for an eye at 



Fro. 29 1. 



o a virtual magnified image a 3 b 3 , and acts therefore 

 precisely like the eye-piece of the common astronomical 

 telescope. 



43. Binocular vision. The Stereoscope. Various 

 visual effects. The same object presents to the eye 

 different aspects when we view it from different points. 

 Thus a six-sided column (fig. 292) appears to an 

 observer on the left like the figure A] to an observer 

 on the right like B. To the former the left face of the 

 column appears broader than the right face ; for the 

 second observer the case is reversed. The same 

 difference in the appearance of objects holds good also 

 for the two eyes of the same person, because the 

 distance between them amounts to several centimetres* 

 If a six-sided pencil is held in a vertical position at a 

 distance of 20 or 25 cm from the eyes, and the right and 



