THE STEREOSCOPE. 



binocular parallax, let him. make two exact drawings of it, one 

 with the right eye closed, and the other with the left eye closed. 

 These two drawings will then represent the object as it is actually 

 seen, when the optic axis of each eye is directed to it. Let us 

 suppose that, by some optical expedient, the two drawings thus 

 made can be so prese'hted'to the two eyes, that the optic axis, when 

 directed to them, shall converge at the same angle as when they 

 are directed to the object itself. In that case each eye will 

 obtain the same view which it would obtain if the object itself 

 were placed before it, and the visual perception must necessarily 

 be the same as would be produced by the object looked at with 

 both eyes open. 



6. Now the optical expedient by which this is accomplished is 

 the stereoscope, a name derived from two Greek words, a-rsptdv 

 (stereon), a solid object, and o-/co7reo>, (skopeo) I look at ; inasmuch 

 as the effect is such as to make the observer imagine that a really 

 solid object (in the geometrical sense of the term), instead of a 

 flat surface, is placed before him. 



Various optical combinations have been proposed and contrived, 

 for the purpose of producing this effect upon two such drawings 

 as we have here described. In some the visual rays proceeding 

 from the pictures are thrown into the requisite direction by 

 reflection, and in others by refraction. 



7. In the first form given to the instrument by Professor 

 Wheatstone, its inventor, the visual rays proceeding from the 

 two pictures were deflected by two plane reflectors placed %t a 

 right angle, so that in entering the eyes they proceeded as if they 

 had diverged from a common point, at which the object repre- 

 sented by the pictures would therefore appear to be placed. 



Fig. 3. 



Let A B c D (fig. 3) be the ground-plan of a rectangular box, open upon 



the side AD so as to admit the light. 

 Let R and L be two eye-holes made 

 in the side B c, at a distance apart 

 equal to the distance between the eyes 

 of the observer. Let E F and p G be 

 two plane mirrors placed at right 

 angles to each other. Let a drawing 

 of an object seen with the right eye, 

 the left being closed, be attached to 

 the inside of D c at r, and another 

 made from the object seen with the 

 left eye, the right being closed, be in 

 like manner attached at I to the 

 inside of A B. Supposing the eyes of 

 the observer to be placed at the holes R and L, the right eye will see by 

 reflection the drawing r in the direction R TO, and the left eye will see the 

 drawing I by reflection in the direction L m. If the lines L m and R n be 

 140 



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