SIGHT. 589 



the lower gains in its power of elevating the ball, which turns 

 upwards and inwards so forcibly as to be brought down again 

 with difficulty by an animal (p. 314-. sq.) ; and, although he had, 

 throughout his paper, ascribed the elevation of the eyes, when 

 closed, to the two obliques (p. 317.), he, at the end of it (p. 332.), 

 ascribes the elevation to the inferior oblique only, and refers to 

 the page in which he had ascribed it to both, as if he had there 

 ascribed it to the inferior alone. 



The straight and oblique muscles, there can be no doubt, are 

 equally voluntary. The straight are clearly for the direction of 

 the eyes, and we become acquainted with the distances, magni- 

 tudes, and positions of objects, by the sensations which accom- 

 pany the motions of the muscles of the eye, or, in other words, 

 from the consciousness of muscular effort. We owe this important 

 principle to Bishop Berkeley, who, in his excellent essay on this sub- 

 ject h , thus expressed himself more than a century ago : " Now, 

 it being already shown that distance is suggested to the mind by 

 the mediation of some other idea, which is itself perceived in the 

 act of seeing ; it remains that we inquire what ideas or sensations 

 there be that attend vision, unto which we may suppose the ideas 

 of distance are connected, and by which they are introduced 

 into the mind. And Jirst, it is certain by experience, that when 

 we look at a near object with both eyes, according as it approaches 

 or recedes from us, we alter the disposition of our eyes, by 

 lessening or widening the distance between the pupils. This 

 disposition or turn of the eyes is attended with a sensation, which 

 seems to me to be that which in this case brings the idea of 

 greater or lesser distance into the mind. Not that there is any 

 natural or necessary connection between the sensation we per- 

 ceive by the turn of the eyes, and greater or lesser distance : but 

 because the mind has, by constant experience, found the different 

 sensations corresponding to the different dispositions of the eyes, 

 to be attended each with a different degree of distance in the 

 object: there has grown an habitual or customary connection 

 between these two sorts of ideas : so that the mind no sooner 

 perceives the sensation arising from the different turn it gives the 

 eyes, in order to bring the pupils nearer, or farther asunder, but 

 it withal perceives the different idea of distance which was wont 

 to be connected with that sensation. Just as upon hearing a 



h An Essay towards a New Tlieory of Vision, sect. 16. sqq. 1709. 



