6 AN ESSAY ON 



means our idea of its outward place is connected 

 with both those sensations, as is manifest by its 

 appearing in two places when its pictures are 

 not painted upon corresponding places of the 

 retinas; which is only a direct consequence 

 arising from our general habit of seeing." 

 Should any one now inquire whence it is, that, 

 to produce single vision, all men agree in di- 

 recting their eyes toward the object in such a 

 manner as to receive its pictures upon corre- 

 sponding points of the retinas, since custom 

 might have connected the sensations of any 

 other two points with the information of its 

 unity from feeling* : This answer maybe given 

 in the words of Dr. Smith t : " When we view 

 an object steadily, we have acquired a habit 

 of directing the optic axes to the point in 

 view; because its pictures falling upon the 

 middle points of the retinas, are then distincter 

 than if they fell upon any other places ; and 

 since the pictures of the whole object are equal 

 to one another, and are both inverted with re- 

 spect to the optic axes, it follows that the pic- 

 tures of any collateral point are painted upon 

 corresponding points of the retinas." 



* This objection is made to Dr. Smith's theory by Dr. 

 Reid, who seems to have overlooked the answer. Reid's 

 Inquiry into the Human Mind, 8vo. p. 332. 



t Vol. I. p. 46. 



