248 Mr. Shaw on the * Sense* of Muscular Action 



merit, it does not interfere with the ray R L'. This ray being 

 admitted, is refracted to the same point, F, in the retina, to 

 which all the other rays, if they had not been shut out, would 



K 



also have concentrated. The ray R I/ being allowed to enter 

 the eye, there is no reason why objects placed above the card 

 should not also have their images represented upon the retina. 

 Let 6 and a be such images, formed by the rays B L, A L. 

 It is obvious that these preserve the same relation to the image 

 at F as they did before the card was used. Now, the true 

 explanation of our seeing this, or, in other words, of our ascer- 

 taining any thing respecting the position of F, is this : there 

 is, first, an impression of the image F conveyed through the 

 optic nerve to the sensorium ; then the image b is presented 

 to it ; and, in succession, the image a comes before it, a pro- 

 cess of comparison, which the mind alone can perform, is in- 

 stituted between these various images, and the result is the 

 knowledge of the relative direction of F to 6 and a. 



It is thus that the surrounding bodies, as well as the parti- 

 cular object of sight, require to be included within the sphere 

 of observation, in order to obtain the idea of ' direction/ As 

 to the intermediate process by which these objects are succes- 

 sively presented to the mind, this is a question which em- 

 braces subjects of the highest interest. 



I would remark, in the first place, that the eye is not a fixed 

 and motionless instrument, and the retina is not possessed of 

 an uniform degree of sensibility throughout all its surface, both' 

 of which things the theory which I have been considering 

 would seem to imply. The retina has one spot in its surface, 



