506 Prof. Wheatstone on the Physiology of Vision. 



on the circumstances under which vision naturally occurs, as 

 preceding writers on this subject mostly have done, but it is 

 necessary to have more extended recourse to the methods so 

 successfully employed in experimental philosophy, and to endea- 

 vour, wherever it be possible, not only to analyse the elements of 

 vision, but also to recombine them in unusual manners, so that 

 they may be associated under circumstances that never naturally 

 occur. 



The instrument I shall proceed to describe enables these ab- 

 normal combinations to be made in a very simple and effectual 

 manner. Its principal object is to cause the binocular pictures 

 to" coincide, with any inclination of the optic axes, while their 

 magnitudes on the retinae remain the same ; or inversely, while 

 the optic axes remain at the same angle, to cause the size of the 

 pictures on the retinae to vary in any manner. 



Two plane mirrors inclined 90° to each other are placed toge- 

 ther and fixed vertically upon a horizontal board. Two wooden 

 arms move round a common centre situated on this board in the 

 vertical plane wbich bisects the angle of the mirrors, and about 

 li inch beyond their line of junction. Upon each of these arms 

 is placed an upright pannel, at right angles thereto, for the pur- 

 pose of receiving its appropriate picture, and each pannel is made 

 to slide to and from the opposite mirror. The eyes being placed 

 before the mirrors, the right eye to the right mirror and the left 

 eye to the left mirror, and the pannels being adjusted to the same 

 distances, however the arms be moved round their centre, the 

 distance of the reflected image of each picture from the eye will 

 remain exactly the same, and consequently its retinal magnitude 

 will be unchanged. But as the two reflected images do not 

 occupy the same place when the pictures are in different positions, 

 to cause the former to coincide the optic axes must converge 

 differently. When the arms are in the same straight line, the 

 images coincide while the optic axes are parallel; and as they 

 form a less angle with each other, the optic axes converge more 

 to occasion the coincidence. When the arms remain in the same 

 positions, while the pannels slide towards or from the mirrors, the 

 convergence of the optic axes remains the same, but the magni- 

 tude of the pictures on the retinae increases as the distance de- 

 creases. By the arrangement described, and which is repre- 

 sented by figs. 1 and 2, Plate XII., the reflected pictures are always 

 perpendicular to the optic axes, and the corresponding points of 

 the pictures, when they are exactly similar, fall upon correspond- 

 ing points of the retinae. The instrument has an adjustment 

 for otherwise inclining them if it be required. 



Let us now attend to the effects produced. The pictures 

 being fixed at the same distance from the mirrors, there is a cer- 



