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W. B. Rogers on Binocular Vision. 325 
partial survey, and this seems to be all that we employ in the or- 
inary vision either of a physical line or a binocular resultant in 
a perspective position. e 
rom what has now been said, it is apparent that the process 
by which we see objects in relief by the binogular combina- 
tion of drawings, is nearly identical with that by which we see 
the objects themselves, but I cannot agree with Sir David Brew- 
axes the action of the eyes is precisely the same whether they , 
are directed to different parts of the binocular resultant or of the 
real object which it represents; but as formerly remarked it does 
not appear that the change of convergency is accompanied in the 
former case by any alteration of refractive power, while in the 
latter case we know that every increase or diminution of the one 
is associated with a corresponding change of the other. 
18. Of binocular direction. 
If on either stage of the stereoscope we place a diagram con- 
sisting of two dots or of two equal parallel lines, and after di- 
recting the eyes upon them so as to see them where and as they 
are, we then by the usual effort cause them to unite into one, we 
perceive that in their approach they move towards an intermedi- 
ate position, and that the resultant dot or line lies in a direction 
midway between those in which the eyes previously saw the two 
pression, derived from the co-existence or union of two different 
feelings of direction furnished by the two eyes. It may there- 
fore I think, be not inappropriately called the binocular direction. 
That in our ordinary vision with two eyes we have a similar 
sense of the binocular direction of objects is I think, proved by 
a variety of familiar facts. Thus for example, in aiming to 
strike an object with a stone or javelin, or in shooting at a mark 
with a bow or a gun held up to the breast, we endeavor to con- 
form the motion which we impress to the feeling of direction de- 
rived from the combined use of the two eyes, and in the game of 
bowls it is this which guides the hand from which the ball is dis- 
charged. Pes aes 
The importance of this peculiar sense of direction, as distin- 
guished from that of the eyes used singly, becomes more striking 
when we consider it in connexion with the perspective resultant 
formed by binocular combination, as in the following examples : 
n each case the combination is supposed to be made on the up- 
per stage of the stereoscope, and, for the sake of clearness, the 
resultant is represented by a dotted line, the arrow head marking 
the nearer or upper extremity. 
