Prof. Wheatstone on the Physiology of Vision. 517 



with the angle nearest the eyes, so that they may be adjusted 

 that their bases may have any inclination towards each other ; 

 and the frame itself is adjustable by a hinge at a, in order to 

 bring the prisms nearer each other to suit the eyes of the observer. 



The instrument being held to the eyes, and adjusted to an 

 object, so that it shall appear single, each eye will see a reflected 

 image of that projection of the object which would be seen by 

 the same eye without the pseudoscope. This is exactly the con- 

 trary of what occurs when the eyes regard the reflected image 

 of an object in a looking-glass ; the left eye then sees the re- 

 flected image of the right-hand projection, and the right eye the 

 reflected image of the left projection, as shown by fig. 6. 



Plane mirrors cannot be substituted for the reflecting prisms, 

 for this reason ; the refraction of the rays of light at the incident 

 and emergent surfaces of the prisms enables the reflexion of an 

 object to be seen when the object is even behind the prolonga- 

 tion of the reflecting surface, as shown at fig. 8, and thus the 

 reflected binocular image may be seen in the same place as the 

 object itself, whereas the images cannot be made by means of 

 plane mirrors thus to coincide. 



When the pseudoscope is so adjusted as to see a near object 

 while the optic axes are parallel, to view a more distant object 

 with the same adjustment, the axes must converge, and the more 

 so as the object is more distant ; all nearer objects than that seen 

 when the axes are parallel, will appear double, because the optic 

 axes can never be simultaneously directed to them. If this in- 

 strument be so adjusted that very distant objects are seen single 

 when the eyes are parallel, all nearer objects will appear double, 

 because the optic axes can never converge to make their bin- 

 ocular images coincide. If the attention is required to be di- 

 rected to an object at a particular distance, the best mode of 

 viewing it with the pseudoscope is to adjust the instrument so 

 that the object shall appear at the proper distance and of its 

 natural size. In this case the more distant objects will appear 

 nearer and smaller, and the nearer objects will appear more 

 distant and larger. 



In ordinary vision, whenever the distance of an object varies, 

 the magnitude of the picture on the retina, and the degree of 

 convergence of the optic axes, always maintain a constant rela- 

 tion to each other, both increasing or decreasing together ; and 

 the perceived magnitude, suggesting to the mind the real mag- 

 nitude of the object, in consequence thereof remains the same. 

 The instrument I described in § 17 shows what illusions arise 

 when the usual relations of these elements of our perceptions are 

 disturbed, by causing one to remain constant while the other 

 varies. The pseudoscope exhibits the still more curious illusions 



