258 SENSE OF SIGHT. 



est ray of the cone, the radiant point will be equally seen in the same 

 direction. Hence, says Sir David Brewster, 1 it is manifest, that the 

 line of visible direction does not depend on the direction of the ray, 

 but is always perpendicular to the retina ; and, as the surface of the 

 retina is a portion of a sphere, those perpendiculars must all pass 

 through one point, "which may be called the centre of visible direction ; 

 because every point of a visible object will be seen in the direction of a 

 line drawn from this centre to the visible point." 



The point o, Fig. 113, is, in Sir David's view, the centre of visible 

 direction. Where a luminous cone proceeds in the direction of the axis 

 of the eye, the centre of visible direction will fall in that line, and a 

 perpendicular, drawn from the point 6, where the rays of the cone meet 

 at a focus on the retina, will pass through this centre of visible direc- 

 tion 0, and the same thing, he conceives, will apply to every other pen- 

 cil of rays. Thus, the rays from D and E, which fall upon the cornea 

 at tj will be refracted so as to impinge upon the retina at s and r re- 

 spectively ; and D and R. will be seen in the direction of lines drawn 

 from these points to the centre of visible direction, o. 



This "law of visible direction" removes at once, Sir David Brewster 

 thinks, every difficulty that besets the subject of the cause of erect 

 vision from an inverted image on the retina. The lines of visible 

 direction necessarily cross each other at the centre of visible direction, 

 so that those from the lower part of the image go to the upper part of 

 the object ; and those from the upper part of the image to the lower 

 part of the object. 



The views of Sir David are embraced by Mr. Mayo, 2 who considers 

 them confirmed by the fact to which reference has already been made 

 that any pressure made upon the retina through the eyeball causes 

 a spectrum to be seen in a direction opposite to the point compressed ; 

 as well as by the following experiments of Scheiner, by whom this law 

 of visual direction was first shown. If the head of a pin, strongly 

 illuminated, be viewed with one eye at a distance of four inches, that 

 is, within the common limit of distinct vision, the object is seen large 

 and imperfectly defined, the outermost cones of rays, which enter the 

 pupil from each point, being too divergent to be collected to a focus 

 on the retina. If a card pierced with a pinhole be now interposed be- 

 tween the eye and the object, the latter may be seen distinctly defined 

 through the pinhole by means of rays that have entered the pupil 

 nearly parallel, with a slightly divergent tendency. But the object 

 may be seen by rays passing either through the upper or lower part, 

 the right or left side, or the centre of the pupil. On shifting the 

 card for this purpose the object appears to move in an opposite direc- 

 tion. Q r? if three pinholes be made, one in the centre, and one at 

 either side, the object appears tripled ; and if one of the side holes 

 be closed, the opposite of the three objects disappears : if, for example, 

 the left-hand pinhole be closed, the right object disappears. Again, 

 if the head of a pin, strongly illuminated, be viewed at the distance of 



1 Op. citat., p. 246. 



3 Outlines of Human Physiology, 3d edit., p. 277. 



