408 W. B. Rogers on Retinal Impressions. 
Tf, while the above adjustment is maintained, we contemplate 
the other image of the pencil situated some distance to the right 
of the lamp, and endeavor to decide, from the mere visual im- 
pression, to which eye it appertains, we almost unfailingly refer 
it 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 illu- 
~ mination 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 snch cases the observation 
is best made in a moderately lighted room by interposing the 
pencil between the eye and a vertical stripe on the wall. 
Exp. 7. Recurring to Exp. 2, in which with a tube in front 
of one eye we perceive a bright circle on the wall in the medial 
direction, we may obtain a pleasing illustration of the point now 
under consideration by bringing 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 
e as in the case of the pencil and lamp, the bright circle 
and the screen are both optically referred to the intersection 0 
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 counteract 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 gts eibomec gx described by Mr. Ward of Manchester, 
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 eprene near the eyes “it becomes as It 
n making this experiment with, a tube of 
aper supported between the thumb and forefinger of the left 
and 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 
suggestions or the intentness with which we fix our gaze upom 
the distant spot to which the axes are converged. 
