IN OPTICS. 87 



and hence we may imagine we see both of them 

 distinctly, though the picture of only one occu- 

 pies that place of the retina, which alone fur- 

 nishes us with accurate vision. 



There are, however, objections to this answer, 

 which seem to me insurmountable. For, in the 

 Jirst place, as the duration of impressions on the 

 retina must be greater or less, according to the 

 vivacity of the pictures which occasion them, 

 it follows, that, were this answer just, the ap- 

 parent field of our distinct vision ought to be 

 in proportion to the quantity of light admitted 

 by the eye ; that it should be contracted, there- 

 fore, by every cloud which passes over us, and 

 be enlarged by every burst of sunshine ; that, 

 at mid-day, it should possess its greatest extent, 

 and ought from that time gradually to decrease 

 till the evening, when its limits should be nearly 

 tjie same with those of the real field of accu- 

 rate vision. Secondly, since the coloured spot, 

 which is produced by looking steadily for some 

 time at aluminous body, appears projected upon 

 every object to which we direct our eyes, dur- 

 ing its continuance, and as such a spot is neces* 

 sarily the sign and effect of the duration of an 

 impression upon the retina ; every other visible 

 appearance from the same cause ought, in like 

 manner, to have its situation determined by 

 the position of the eye, as far as this may be 



