Adjustable and Reversible Stereoscope. 323 



vision, and to reduce to the utmost the difficulty usually atten- 

 dant upon stereoscopic vision by the latter method. 



VI. To secure the means of producing stereoscopy from 

 perfectlv similar pictures, by making the retinal images of 

 these dissimilar by oblique vision. 



It gives me pleasure to acknowledge my indebtedness to 

 Messrs. E. and H. T. Anthony and Co., of this city, for every 

 facility that could be asked in testing such devices as pre- 

 sented themselves to my mind : and the form finally adopted 

 gives entire satisfaction, the instrument being compact, inex- 

 pensive, and easily managed. 



The semi-lenses, I, I (fig. 6), instead of being cemented to 

 wood or brass, are gently pressed into a pair of boxes, in the 

 opposite walls of which square openings have been cut to 

 transmit the light from the stereograph, which is held on the 

 cross bar, a, bv pressure between a pair of brass springs, and 

 may be adjusted to any desired position by sliding this along 

 the longitudinal bar. A pair of adjusting-screws, s, s, come 

 into contact with the bases of the semi-lenses, and are opposed 

 bv a pair of springs, against which the thin edges press. If 

 the semi-lenses are thus pushed together as near as possible, 

 the ravs which enter the observer's eyes pass through the 

 thicker parts, and hence are but little deviated. If they be as 

 far apart as possible, the increase of deviation makes it con- 

 venient to employ a stereograph on which the interval between 

 corresponding points is greater than on those generally pre- 

 pared. This interval is ordinarily limited to about three 

 inches. The focal length of the semi-lenses being seven inches, 

 it has been found that a stereographic interval of four inches 

 can be allowed without any very objectionable coloration due 

 to lack of achromatism, the visual lines of the observer being 

 now parallel or slightly divergeat. This divergence mav be 

 increased to any extent that he is willing to endure, bv turn- 

 ing the adjusting-screws, so as to press the semi-lenses closer 

 together. Binocular fusion of images is thus retained : but 

 the associated contraction of the external rectus muscles pro- 

 duces the illusion of slight recession of the object viewed and 

 increased depth of relief. Similar effects may be obtained bv 

 using conjugate pictures separated by the ordinary stereogra- 

 phic interval: but they are much less noticeable, because there 

 is less disturbance of the ordinary condition of the muscles. 



In front of the partition between the lens-boxes is hinged a 

 pair of narrow brass screens, c, c, which, in the experiments 

 just described, are pressed flat as in fig. 6. The left picture 

 is thus hidden from the right eye and the right picture from 

 the left eye. By folding them together, as in fig. 7, neither 



