242 Mr. Shaw on the < Sense ' of Muscular Action 



' the image of the point at F, fall upon the retina in all possible 



* directions from L/ F to L F ; and we know that the point F is 



* seen in the direction F C R. In the same manner the points 

 '//' are seen somewhere in the directions /' S,/T. These 



* lines, F R, /' S, / T, which may be called the lines of visible 



S. 



K 

 T 



T 

 T 



' direction, may either be those which pass through the centre 



* C of the lens L/ L, or, in the case of the eye, through the centre 

 ' of the lens, equivalent to all the refractions employed in pro- 

 ' ducing the image ; or it may be the resultant of all the direc- 

 ' tions within the angles L/FL, I/yL; or it may be a line 

 ' perpendicular to the retina at F/'/. In order to determine 

 ' this point, let us look over the top of the card at the point of 



* the object whose image is at F, till the edge of the card is just 



* about to hide it ; or, what is the same thing, let us obstruct all 



* the rays that pass through the pupil excepting the uppermost 

 R L' ; we shall then find that the point whose image is at F 5 

 4 is seen in the same direction as it was seen by all the rays I/ F, 

 C F, L F. If we look beneath the card in a similar manner, 



* so as to see the object by the lowermost ray R L F, we shall 



* see it in the same direction/ 



It is remarkable that Dr. Brewster, in conducting an argu- 

 ment of this nature, should have fallen into two such errors as 

 are here exhibited. 



1. He endeavours to demonstrate how the retina can dis- 

 cover the relative direction of an object, and yet he omits 

 altogether to present any second object or image to our notice. 

 In each of the three instances which he has given, he has 

 confined his attention solely to the object whose direction forms 

 the question at issue, or to the rays which come from it, or to 



