EXPERIMENTS, &c. 



The question I have just treated, naturally 

 gives rise to another : Would it have been 

 more to our advantage, if accurate vision, in- 

 stead of being confined to one point of the 

 retina, had been possessed by every part of that 

 membrane? I answer, I think not, for the fol- 

 lowing reasons. 



First ; The diffusion of such a property over 

 the whole retina would be of little use, unless 

 our power of attention was also increased. For 

 we should otherwise be still unable to perceive 

 more than one visible object at once, with di- 

 stinctness, since, by our present constitution, 

 we are capable of attending accurately to only 

 one thing at a time. The only benefit, indeed, 

 I can see to arise from such a condition of the 

 retina, is this; That our attention might be 

 shifted more quickly from picture to picture on 

 that membrane, than our eyes can be turned 

 from one external object to another. This ad- 

 vantage, however, would be far out-weighed by 

 an inconvenience accompanying it. For it is 

 a well-known fact, with respect to perception, 

 that we are capable of attending, more or less 

 accurately, to any particular impression upon 

 the senses, in proportion to the inferior force of 

 other impressions, which are at the same time 

 received. But in the supposed state of the 

 retina, there would be, almost always, several 



