276 Transactions of the V^T^Slf^i^: 



may be used without any bar by princiiDle, provided only tbat the 

 optician has skill enough to split it without injury, and to remount 

 its halves without deranging its adjustments. And this is a more 

 simple matter than would at first sight appear. 



The bisected tube is mounted in such a manner as to allow the 

 motion to focus to be given to the stage, the body having peculiar 

 adjustments of its own. 



In adjusting to distinct vision, it is necessary that each half 

 should move the same quantity to and from the object at the same 

 time, in order to be in focus at the intersection of the optic axis. 

 This points to the necessity for an adjustment for point of con- 

 vergence of visual rays. 



Now, according to the difieriug distances between the eyes of 

 different persons, the half of the body must be capable of being 

 drawn wider apart, or of being brought closer together, to ensure a 

 single solid field. 



This is the adjustment for visual angle, and on it depends much 

 of the beauty of the stereoscopic effect. 



Further, it will be conceded that the perfection of vision of 

 every kind in all cases governs the construction and arrangements 

 of all instruments, and although it would seem at first sight im- 

 l^roper to make binocular vision with two instruments of diflering 

 powers, yet if a more equable and soHd picture can be so observed, 

 it shows that the natural focal length of each eye is not identical. 

 Therefore I introduce an Adjustment for Inequality of Visual Foci. 



These principles are carried out mechanically in a very simple 

 manner : Two metal bars are jointed together at the bottom, and the 

 joint is fixed somewhere in the plane of focus of the object-glass, 

 and the halves of the divided body sHde to or from the joint of the 

 bars by corresponding attachments. 



Part of a rotation of a Mill, connected with the top of the bars, 

 separates the halves of the body to the distance between the eyes, 

 so adjusting for Visual Angle. 



A second mill is placed near the bottom of the body, and con- 

 nected by a crank and link to the third mill, which is capable of 

 motion in a vertical slot, and connected to each half of the body by 

 a link ; so that if the second mill is turned partially round, it will 

 raise or lower the third in its slot ; and this, acting through its links, 

 raises or lowers the two halves of the body equally, and so adjusts 

 for Point of Convergence of Visual Rays ; or, in other words, makes 

 the plane of Horopter coincide with that of the Focus. 



Now, if the third mill, while remaining motionless in its slot, be 

 partially rotated, it will raise one half of the body and depress the 

 other half, or vice versa. Thus we can lengthen or shorten the 

 focus of either half of the object-glass, and completely accommodate 

 any Inequality of Visual Foci between two eyes. 



