NATURAL PHILOSOPHY. 153 



adjustment and motions we are unable to determine which is actually visible 

 to the right and which to the left eye. 



The same experiment furnishes also an incidental illustration of the prin- 

 ciple of transposed visual reference, before alluded to. If, while the above 

 adjustment is maintained, we contemplate the other image of the pencil, sit- 

 iiated some distance to the right of the lamp, and endeavor to decide, from 

 the mere visual impression, to which eye it appertains, we almost unfailingly 

 refer to the right eye as that which most nearly fronts it; although obviously 

 it belongs to the other, as will be found at once on closing either eye. 



Where the eyes are externally very sensitive, any strong illumination of 

 one as compared with the other will interfere with the effect above described, 

 by referring the impression specially to the eye thus unduly excited. In such 

 cases the observation is best made, in a moderately lighted room, by inter- 

 posing the pencil between the eye and a vertical stripe on the wall. 



Exp. 7. Recurring to experiment 2, in which, with a tube in front of one 

 eye, we perceive a bright circle on the Avail in the medial direction, we may 

 obtain a pleasing illustration of the point now under consideration, by bring- 

 ing a dark card or book, or even the hand, between the uncovered eye and 

 the wall. The spot, instead of being intercepted, will appear as a perforation 

 in the opaque screen. 



Here, as in the case of the pencil and lamp, the bright circle and the screen 

 are both optically referred to the intersection of the two lines of view. But 

 the luminous circle almost or entirely obliterates the corresponding part of 

 the screen. As the full view of the screen and its connections continually 

 remind us that it is in front of the uncovered eye, we are led to refer the 

 luminous circle seen as coincident with a part of it to the same eye, and 

 thus to believe that we are looking through the screen with that eye. It is, 

 however, not difficult, by intently regarding the luminous circle, so to coun- 

 teract the force of this extraneous suggestion as to feel, even in this case, as 

 if the circle were equally in view to both eyes. 



These considerations explain very simply the experiment of the pseudo- 

 diascope described by Mr. Ward, of Manchester, which, like several of those 

 above mentioned, is but an instance of the old observation of Da Vinci, that 

 when we see behind a small opaque object presented near the eyes " it be- 

 comes as it were transparent." In making this experiment with a ttibe of 

 paper supported between th thumb and fore-finger of the left hand, and held 

 before the right eye, so that the back of the hand may be some inches in 

 advance of the left eye, it will be noticed that the effect varies with the amount 

 of convergence of the eyes, and that the bright perforation in the hand may 

 or may not be referred to the left eye, according to the force of the accessory 

 suggestion, or the intentness with which we fix our gaze upon the distant 

 spot to which the axes are converged. 



In conclusion, it may be remarked that the experiments which have been 

 described are, for the most part, too obvious and familiar to have merited 

 such a special notice, but for the peculiar and in some respects new interpre- 

 tation which they have offered of many visual phenomena. Considered in 

 this relation, we are, I think, entitled to conclude from them, 



First, That the retinal impression of an object presented directly to either 

 eye is accompanied by the feeling of a united visual act, and of itself gives 

 no indication of the particular eye impressed. And, 



Second, That the reference of the impression to one eye rather than the 

 other is the result of collateral suggestion, which may either locate the image 



