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. 



