THE STEREOSCOPE. 



Fig. C. 



convexity of the lenticular eye-pieces A E and c E', may be such 

 as to produce 

 any desired mag- 

 nifying effect, 

 within practical 

 limits, upon the 

 two pictures. 



The tubes con- 

 taining the eye- 

 glasses A E and 

 c E', are made to 

 draw in and 

 out so as to be 

 adapted to dif- 

 ferent eyes ; and 

 they are fixed 

 by pins, which 

 pass into slits 

 made in them 

 in that position 

 in which the de- 

 flected rays have 

 the proper degree 

 of divergence. 



The form in 

 which this lenti- 

 cular stereoscope is usually constructed, is shown in fig. 7. The 



pictures are either opaque or 

 transparent. If they are opaque, 

 they are illuminated through an 

 opening A B c D, covered by 9. 

 hinged lid, the inside surface of 

 which is coated with tinfoil so as 

 to reflect light upon the pictures. 

 If they are transparent, the base 

 of the instrument A' B' c' D' has 

 a plate of ground glass set in it, 

 which allows a diffused light to 

 pass through the pictures. 



9. In what has been stated 

 above, it has been assumed that 



two drawings of the same object can be produced, differing one from 

 another precisely as the two views of the same object would differ, 

 when viewed by the right and the left eye successively, subject 

 to a given degree of binocular parallax. Now, the difficulty, if 

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