430 INSENSIBLE SPOT OF RETINA :-VISUAL ATTENTION. 
as well as of its relations to the rest. It will be presently 
shown that when we employ both eyes at once, their axes 
meet in the object, and that the degree of their convergence 
affords us a very important means of judgment as to their 
distance (§§563, 564).—The part of the retinal surface which 
lies over the entrance of the optic nerve, is remarkable for the 
imperfection of its power of receiving impressions ; as is made 
apparent by the following experiment. Let two black spots 
be made upon a piece of paper, about four or five inches apart; 
then let the left eye be closed, and the right eye be strongly 
fixed upon the left-hand spot; if the paper be then moved 
backwards and forwards, so as to change its distance from the 
eye, a point will be found at which the right-hand spot dis¬ 
appears, though it is clearly seen when the paper is brought 
nearer or removed further; and it can be shown that in this 
position of the eye and the object, the rays from the right- 
hand spot fall upon the point in question. 
555. The degree in which the attention is directed to them, 
has a great influence on the readiness with which very minute 
or distant objects can be perceived; and there is a much 
greater variation in this respect amongst different individuals, 
than there is in regard to the absolute limits of vision. Many 
persons can distinctly see such objects, when their situation 
is exactly pointed-out to them, who cannot otherwise distin¬ 
guish them. There must be few who have not experienced 
this, in regard to a balloon or a faint star in a clear sky, or a 
ship in the horizon; we easily see them after they have been 
pointed-out to us; but if we withdraw our eyes for a few 
minutes we search in vain for them, until they are again 
pointed-out to us by some one who has been watching in the 
interval. The faculty of rapidly descrying such objects much 
depends upon the habit of using the eyes in search of them; 
thus a seaman will distinguish land, when to the landsman 
there is no appearance more distinct than that of a faint cloud 
on the horizon presenting no definite outline; or he will 
recognise the course and rig of a distant ship, which to the 
landsman appears but as a white speck on the ocean : yet the 
landsman, placed before a piece of delicate machinery, might 
be able to astonish the seaman by distinguishing the forms 
and movements of minute parts, which to the latter appear 
only as a confused mass. 
